Korosensei

"The tentacles spoke to me, asked what I wanted to be. To be weak I said, to be flawed, vulnerable. To perceive weakness in others not to exploit but to protect. To be a guide, to be a teacher."

- Korosensei's goal

Korosensei is the homeroom teacher of Class 3-E of Kunugigaoka Junior High School, and the central character and main antihero of Assassination Classroom. He claimed to be responsible for creating the permanent crescent moon, and added that he planned to destroy the earth after teaching Class 3-E for a year.

Prior to being a teacher, he was the assassin known as "The Reaper".

Appearance
Korosensei has the appearance of a yellow bipedal octopus. He is about three meters tall and possessed a large, bulbous head with a large smile that rarely breaks or moves and small, beady eyes. He uses two tentacles as arms, each having two feelers as fingers, and utilized the rest of his tentacles to walk. Korosensei has a pair of hidden nostrils, which make him appear to have four eyes if detected. Due to his constant grinning, he has the ability to change his color depending on his emotions.

Korosensei wears a black academic dress, a small black squarish academic cap with a yellow tassel, and a large black tie with a yellow crescent-shaped moon on it, which is a gift from Aguri.

As a human, he had pale skin, a slender build, dark eyes, and short black hair that reached to his nape. He sports an amicable smile, which was initially used to manipulate others. When carrying out assassinations, he donned formal clothing. After being taken prisoner by Yanagisawa, he was put in patients' scrubs until his escape and transformation.

Personality
It was revealed that he was raised in an environment "where everybody could easily betray one another to the point that one could only trust oneself". Through these hardships, he decided to become an assassin in order to survive. Despite gaining an apprentice, he only saw him as a tool for his goals, which drove the latter to finally betray his master.

However, after meeting Aguri Yukimura, he finally knew what compassion really was--becoming much more kind and compassionate. According to Aguri, if he was born in a peaceful country, his personality would be "someone a little perverted, with a good head but a few screws loose, petty about small things, and also very stubborn." He has/would have "a gentle smile not just for business, but because he is a gentle person himself." This prediction happens to reflect his personality; which became forefront after he answered the tentacles.

When first introduced, Korosensei was a jovial individual who demanded absurd and seemingly mundane requests despite claiming he would be the cause of the planet's destruction in a year. He was also rather kind to others in general and could switch from silly and childish to serious on the fly. According to Nagisa, Korosensei also was unintentionally petty at others at times and loved gossip, both traits heavily noted throughout the series.

From time to time pieces of his old personality as The Reaper were visible such as when Terasaka tried to sacrifice Nagisa to kill Korosensei and Korosensei menacingly threatened all of their families if they tried another self-harming attempt like that. In that moment he resorted to his old method of teaching through fear. As the series progresses his character development renders him more and more capable of true empathy and learning to use that empathy to teach lessons instead of through fear and reward like he had with his first student.

Korosensei deeply cared for his students' emotional and physical well-being and would lose his temper whenever any of them were put in danger, even by one another. On the annual school trip to Kyoto, a few of his students were assaulted/kidnapped by a gang of high schoolers hidden in an alley. After searching the entire area painstakingly, he infiltrated the abandoned building where two kidnapped students were being held at and single-handedly beat up all of the high schoolers, scaring them away.

One of Korosensei's attributes was the fact that he was able to connect to each student individually, being an objective, empathic, and observant person who will do almost anything to help his students. This was the reason why all the students respect him despite the fact that he was going to blow up the Earth in a year. Taking care of each student's needs, Korosensei would sometimes be piqued by a student's current problems or interests and would try his best to guide the student along or solve it for them.

Korosensei loved sweets almost to extreme cases, and was also shown to be a very perverted character, a trait he shares with his student Okajima. This characteristic was revealed to be the side effect of the experimentation which made him unable to hide his true feelings.

Etymology
His name, Korosensei, is a combination of the words 殺せない (korosenai), which means "unkillable", and the word 先生 (sensei), which means "teacher". It was given to him by Kaede Kayano.

History
As a child, he was born into one of the vilest and corrupt slums in an unknown country. It was an environment where everybody could easily betray one another to the point that one could only trust oneself. Through these hardships, he decided to become an assassin, eventually becoming the master assassin known as the The Reaper. Immediately, his talents were shown as he left a tremendous trail of corpses in his wake. He became notorious for breaking through even the most strict security teams to assassinate his targets without any hitch.

All this changed after assassinating a wealthy businessman and father. The man's child witnessed the killing and the child became enamored with the beauty of the kill, requesting that the Reaper train him as his apprentice. Eager for the assassination opportunities that an apprentice could give, the Reaper took in the boy and trained him. He taught him everything he knew all while still instilling respect and fear in him and letting him know the difference in power between them, though he only saw him as his tool rather than a real student.

After several successful jobs, the apprentice turned on the Reaper as he was attempting to escape a prison facility. The apprentice, now the current Reaper, sought his master's title and abandoned him just to fuel his ego. Betrayed, the Reaper was captured and thrown into an undocumented research laboratory run by Kotarō Yanagisawa. Yanagisawa's research goal was to create large amounts of antimatter by having it "piggyback" on the growth of human cells.

It was there that he met and befriended Aguri Yukimura, a teacher who was at the time in an arranged and abusive marriage to Yanagisawa. She was brought in to keep a closer eye on the him as Yanagisawa considered her easily expendable if he attempted to break free and take her hostage. As the tests continued, the Reaper began to slowly bond with her and became fond of her as she worked on compiling exam questions for her class, and it was during this time he learned what compassion was. However, a problem arose when Yanagisawa and his researchers began to question what would happen to Korosensei's antimatter cells when he died. To answer this question, Yanagisawa and his team launched a small automated lab onto the moon to safely test the possibility of what happened if a subject with Korosensei's antimatter infused cells were to die, in this case, a small lab rat. When the rat did die at the end of its lifespan it caused a volatile reaction with the antimatter in its body. The antimatter created a chain reaction that turned 70% of the moon's matter into antimatter, which then completely exploded. Upon seeing this, Yanagisawa was horrified at the results. His subordinates then began running calculations and discovered that the same thing will happen to the Reaper by March 13 of the following year. Earth would be destroyed the same way if left unchecked, prompting Yanagisawa to order his subordinates start preparations to kill him. A mortified Aguri overhears this and decides to go speak to the Reaper and tell him their findings. When she does however, he reveals that it does not bother him as he expected a death such as this from the start. Telling Aguri to leave, he finally unleashes his full power that he gained from the experiments to escape as a visibly fuming Yanagisawa watches from a nearby security camera. The Reaper then proceeded to rampage violently, killing off several guards and in the process damaging Yanagisawa's left eye.

However, during the escape Aguri is mortally wounded when she landed in the line of fire of a special tripmine meant for him to the latter's horror. He then laments the fact that if he hadn't wasted his skills on killing, he would know how to save her. Her final wish to him was to teach the students of Class E all while thinking of what a wonderful teacher he will be. With this, she also left a large tie as a present for him to take. Depressed but not broken, the Reaper vowed to help those children. He left a note then left the facility for a nearby mountain, and then uses various items on the mountain to create a new set of clothes to suit his new role as a teacher.

Although not responsible for destroying seventy percent of the moon, he takes the blame and tells the world's leaders that he will destroy the Earth as well in March of the following year and requests to become the homeroom teacher for Kunugigaoka Junior High School's Class 3-E. The world leaders accept the deal, hoping it will give them a chance to assassinate him; however, they also added another condition: he would not harm the students.

Introduction: Assassination Time
As Korosensei starts the attendance, he asks the one who is on day duty to issue command. Korosensei was then ambushed by his students with automatic rifles; however he dodges the bullets while doing the attendance. Korosensei is pleased that that there are no absentees; he was also ashamed that nobody could hit him. He tells them that they should devise a plan together. The students complain that even if he were to get hit by the BB pellet bullets, he would just endure it. Korosensei demonstrates that those BB bullets, created specially in America for harming him, are harmless to the students, and shoots his right feeler off; however, Korosensei regenerates his feeler back after a few seconds. He warns them not to use them indoors except for killing him. He then tells them that they will be able to kill him before graduation, and decides to tidy the class up.

Later he questions Kimura of which one of his four tentacles is the weird one, which Kimura responds that the blue tentacle is the odd one, and Korosensei is then pleased with his correct answer. He then catches a bullet with his chalk. He becomes red and punishes Nakamura, saying that her assassination meddles with her study. When lunchtime starts, Korosensei decides to go to China, while grading his student's tests. Korosensei later came back with a missile souvenir after having been ambushed by the self-defense in the Sea of Japan. Nagisa asks him if it was troublesome that he was targeted, but Korosensei tells him that he doesn't mind, and that it shows proof of his vast power.

The class starts again, and Korosensei decides to assign a poem, and the one who is finished should bring it to him. Korosensei was asked by Kayano what name they should refer him as; Korosensei tells her that they can decide. Korosensei sees Nagisa walking up deliver the letter. He was suddenly struck by him, but he easily blocked it. However, Nagisa retaliates with a BB pellet grenade, much to Korosensei's surprise; however Korosensei uses his shed skin to protect himself and Nagisa, which he can only use once a month. After revealing his ability, he turns black with extreme anger, and calls out the ringleaders of the BB pellet grenade. Korosensei then roams around town with his mach 20 speed and brought the nameplates from their houses. He tells the students: due to the agreement to not harm the students, if they try the same assassination method again, then he would annihilate anyone they know. He was questioned by Terasaka of the degree of the assassination, and Korosensei corrects him by saying that it was excellent and praises Nagisa for keeping a natural body movement. However, Korosensei admonishes them of having no regard for Nagisa's life. He then advises them that they should use an assassination method that will make everyone happy and that they should use their hidden talents. After seeing a burn mark on a table he remembers the incident from his past.

Inspiring his class
Apart from avoiding every assassination attempt by the class, Korosensei teaches them valuable life lessons and give the individual students the confidence in themselves despite being placed in the discriminatory Class 3-E. When going up against the school's discriminatory education policy led by the board chairman Gakuhō Asano, Korosensei lost in the mid term exams by having been focused too much on getting all of Class E ranked in the top 50. At the time however, it was also to teach the students the importance of "having a second blade", since in the event Korosensei is killed, the students still have to plan for their future in life regardless of who claims the ten billion yen bounty. During the exhibition match between Class E and the school's baseball club, Korosensei coached the class in countering the star player's pitches and helped them win despite Asano's interventions. While the students are grateful that their teacher is doing all of this for their benefit, they still get annoyed by his various antics, such as eavesdropping for juicy gossip and getting too cocky, which normally gets Korosensei into trouble.

During the semester, Korosensei had been challenged to a duel against the second transfer student Itona Horibe, accompanied by the mysterious Shiro. To his and everyone's surprise, Horibe has the same tentacle implants as Korosensei which explains why Horibe had earlier called Korosensei his 'brother'. Although visually disturbed by Itona's tentacles and the well planned strategies devised by Shiro, Korosensei outmaneuvers the pair and defeats Itona. Before Korosensei could get any answers regarding Itona's tentacles the pair retreated but not before vowing to return. The pair kept their vow and returned to ambush Korosensei but he was rescued by the students who were none too pleased that they were used and almost killed indirectly by Shiro.

End of Term Time
With the finals approaching Korosensei motivates the class into getting first place in any of the five subjects, with the reward being that they have the right to shoot off one of his tentacles. Knowing that this will slow down his speed, the class takes up the challenge enthusiastically, but also got drawn into a wager with Class A. The results show that Rio Nakamura, Yūma Isogai and Manami Okuda scored first place in English, Social Studies and Science respectively. However Korosensei has been conned to have four additional tentacles destroyed when Terasaka, Hazama, Muramatsu and Yoshida tied for first place in Home Economics, a subject Korosensei tried unsuccessfully to disregard as not being a core subject. The class's plan is to attempt their ambitious assassination on a holiday island resort in Okinawa, which was the prize for winning their wager with Class A.

Assassination Island Arc
Arriving on the island resort, Korosensei participates in many activities with the class seemingly not knowing the true purpose of his students' agenda ahead of the assassination. At the end of the day Korosensei has been sun burnt to the point that no one can read his burnt black face. He sheds his skin as his monthly routine but unwittingly weakens himself and wasted one of his defense measures. The class then makes him watch an embarrassing film of his antics and unusual hobbies as a way to mentally weaken him. When it finishes he realizes too late of the high tide as the seven shoot off his tentacles. From there, his surroundings change dramatically as the students trap him in a hydraulic water cage and fire anti-sensei BBs all around him. As he senses two BBs fired by Chiba and Hayami, Korosensei was forced to use his last resort, turning himself into a ball with an unbreakable shield surrounding him at the cost of forsaking his mobility.

Later that night, 10 of his students collapse to an artificial virus. The mastermind behind the virus puts them to ransom and demands that Korosensei be delivered by Nagisa and Kayano to the mountain hotel in exchange for the antidote. Although unable to move, Korosensei assists the 15 remaining students to infiltrating the hotel, and even helped call out commands while simultaneously distracting one of the hired hit men. When the class discover the mastermind to be their short-lived PE teacher Akira Takaoka, Korosensei is forced to watch him and Nagisa battle it out for the antidote. The situation was so grave that Korosensei grew worried and even advised Karasuma to intervene should anything bad happens. Fortunately Nagisa was able to defeat Takaoka and the ill students had never been in any danger; the virus being a fake. The next day, Korosensei was sealed away in concrete slabs by Karasuma in a vain effort to kill him but he manages to return to his normal form.

Before leaving the island, Korosensei organised a Test of Courage. It is supposedly to be a haunted attraction but his ulterior motive is to create more juicy gossip by pairing up the students to spark any romances. The class immediately realizes what Korosensei had planned, and the later half of the pairs instead turned the tables on their teacher by making him scared.

Continuing to inspire
Much like the first semester Korosensei continued teaching Class 3-E some valuable life lessons. He gave the class some practical experience to their newly-learned ability to free-run taught by Karasuma by organizing a game of Cops and Robbers, and inspired Yūma Isogai to lead his class to victory against Class A during the Athletic Festival.

Korosensei briefly suffered a loss in trust with his students when they suspect him of crimes he did not commit due to his perverted hobbies. He was vindicated when he and a few of the sceptical students find the culprit to be Shiro and Itona, who once more ambush him using the same strategy employed by Class 3-E during the summer vacation. This time, Korosensei was able to convince Itona to join them after the latter was coldly abandoned by Shiro for the repeat failures, and removed the tentacles from him.

Unwanted attention
As the mid-terms approached, some of the students started to take short cuts by studying and training at the same time, free-running through the town despite being expressly forbidden to do so. This soon gets them into trouble when they surprise an old man and cause him to break his leg. Korosensei was furious upon learning of this, and slapped the students involved in his anger despite the condition he was not to harm them. To teach them humility Korosensei forbids them to study for two weeks and help out at the nursery the old man runs, which consequently lowered their test ranks, save for Karma.

The students' actions had also attracted the attention of the professional hit man known as the The Reaper. When Korosensei was away, the hit man took the class hostage to lure the target in and Korosensei was soon captured. They were later freed when Karasuma defeated the Reaper.

Nagisa's Time
Afterwards, Korosensei held a careers consultation for the class, wishing to know what their second blade will be should somebody succeed in killing him, while humorously avoiding each student's attempts to kill him. After hearing Nagisa's admission that he has the talent to become an assassin, Korosensei vows to support him despite the dangers of this path. He later disguised himself as Karasuma during a parent-teacher interview with Nagisa's mother Hiromi who wants to transfer her son back to the main campus. But seeing her dominant personality and inferiority complex to make Nagisa follow the path in life she laid out for him, Korosensei refuses her request. That night he saw Nagisa take out a hidden assassin in front of his mother and drove them both back to their house.

Final Battle with Chairman Asano
During the school festival, Korosensei advised the students to use the mountain to find natural ingredients to make acorn noodles as a way to attract customers to travel the one kilometre trek from the main campus to the 3-E classroom. In his final battle with Chairman Asano, Korosensei aided his students to triumph over Class A, and even turned the tables when Asano himself arrived to kill him. There Korosensei revealed that he based his teaching methods based on Asano's own methods in his early teaching career, and was responsible in Asano somewhat regaining his former kind personality.

End of the Semester: Past Time
The end of the semester took a dangerous twist when Kaede Kayano launched a surprise attack on Korosensei, revealing to have implanted tentacles on herself. Korosensei managed to avoid falling into a anti-sensei BB-filled pit by blasting a hole in the wall, prompting Kayano to recoil on instinct. Korosensei asked what happened to her afterwards, and she reveals to be the younger sister of Aguri Yukimura, drawing a surprised reaction from him and her classmates. At the students' insistence and seeing his past finally catching up to him, Korosensei agreed to tell the full story of his past, but not until Kayano is subdued.

He and the class meet her at Kunigigaoka Park, and the two start to fight. Immediately Korosensei senses Kayano's subconscious fading as her tentacles start to dominate, and asks the class to find a way to calm Kayano's bloodlust before she dies. Korosensei intentionally reveals his main weak point - his heart behind his crescent moon tie - and Kayano strikes. Although being gravely wounded, Korosensei pins her down stating his promise to her sister not to let any of his students fall, allowing Nagisa to approach Kayano and kiss her into unconsciousness. Korosensei removes her tentacles and sees Shiro (who Korosensei now recognizes to be his jailer Kotaro Yanagisawa) watching the whole thing with a masked accomplice. When Kayano woke up filled with regret, Korosensei tells the class about his past as the original Reaper, bonding with Aguri during his captivity and promising to teach her students before she died during his escape. He also stated that even if no one managed to assassinate him by March, he would die along with the entire world.

Division Time: The Civil War of Class E
At the start of the final semester Korosensei greets his class with usual enthusiasm. But he notices their unenthusiastic response even after the winter break stemming from hearing his past. While Karasuma is worried that their motivation to kill is weakened, Korosensei only states of how the class will respond. The teachers later arrive after seeing a student meeting taking place but degenerates into a violent scuffle between Nagisa and Karma. Korosensei mediates the situation by suggesting that the two factions (those wanting to kill Korosensei and those wanting to save him) settle their differences by playing a paintball game, with the winning side having their opinion to be the final say in uniting the class again.

Korosensei later watches the battle unfold from a raised platform, and commentates on some of the individual students' hidden talents. When the fight is reduced to the last man standing between Nagisa and Karma, he compares each of the boys' specialized skills. Eventually Nagisa wins when Karma surrenders voluntarily but the class has been reunited under a common goal once more. Karasuma later speculates that the war is what Korosensei had planned all along, another lesson to educate the students.

Renewed Resolve
Searching for a way to save their teacher, the class eventually come across information that could meet their purpose. The information is based on American research data experimenting how to neutralise the antimatter without causing it to explode. However the experiments are being conducted on board the International Space Station by trained personnel. Korosensei suggests a crazy plan to the students to infiltrate a Japanese launch site and have two students replace the dummies in the shuttle and essentially hijack the space station. At the site the class quietly infiltrates the facility, and delegate Nagisa and Karma to be the hijackers. As the two launch into space, Korosensei barely manages to keep up with the launched shuttle as he wishes the pair luck.

The boys return to Earth with the data and Korosensei helps Ritsu steer the shuttle back to their classroom. After bribing an irritated Karasuma with the successful experiment of sending two living beings into space without any harm done, he soon learns from the research data that the chances of him self-destructing in March is less than one percent. Moreover, the chemical he and Okuda worked on back in the first semester that liquified him also neutralises the antimatter's effects. Despite learning of this, Nagisa and the class resolve to at least continue their mission to kill their teacher until graduation. Hearing this, Korosensei throws a quick-fire spontaneous round of festivities the class did not experience during the winter break due to their depressed state after hearing his past. While he celebrates, the class made numerous attempts to kill him but their target's speed and their own sloppiness meant their efforts were for nothing. The teacher soon brings the class back down to reality by announcing the high school entrance exams in two weeks' time, and that he will hold another careers' consultation afterwards to discuss the students' futures after graduation.

Return to shenanigans
When the entrance exams arrive, Korosensei wished every student good luck with his usual enthusiasm (unwittingly embarrassing them in the process). Afterwards he tried to console a devastated Takebayashi who bombed out in getting to the top high school as his first preference. He also punished every student that uttered any words synonymous with 'fail' with an outdated hairstyle known as 'seven-three'. The class eventually realise that their target was baiting them purposefully to entertain himself and try to kill him in anger.

A day before Valentine's Day, Korosensei influences Maehara to spend time with Hinata Okano as a way to catch more love gossip. Maehara took his advice but also knew of Korosensei's motives to spy on them and had some of the class wait in ambush. This failed but Okano was especially infuriated at not being told of this plan, which made Maehara ending up trying to reconcile with her the next day. Korosensei spent much of the day spying on the class for love gossip, but was easily distracted by Kayano's photo of Aguri in a swimsuit and the chocolate present, preventing him from taking pictures of Kayano and Nagisa. During night time when he was continue to admire Aguri in a swimsuit photo at staff room, dodged Chairman Asano's sudden attack and was offered the choice to continue teaching after March; however Korosensei declined and they are chatting together for the whole night.

With the whole class having been accepted in their top preferences for high school, Korosensei wanted to collate a graduation photo album using the 10,000 photos he had already taken, most of them being the students' private or embarrassing moments. Believing this to be not enough he then gathered the class for a quick round-the-world tour, taking snapshots and leaving without any chance for sight-seeing. Afterwards, he met the students individually for another careers' consultation and met Nagisa last. He was happy that Nagisa decided to become a teacher and supported his decision. After Karasuma leaves the classroom with Korosensei compiling his graduation photo album, a weaponised beam of light struck the classroom.

Assassination Time, 2nd Hour
Much to the government's surprise, Korosensei escaped the space weapon dubbed the "Spear of Heaven" but not unscathed. He tried to fly away but was blocked by the impenetrable "Earth Shield", which also extended underground. Trapped on the mountain, Korosensei was impressed and seemingly admitted defeat. He continued to work on the graduation album though he wished to see his students one last time.

On the final night of the deadline, Korosensei and the class happily reunited. Briefed of the situation and that the satellite weapon would fire again in 90 minutes, he imparts with one last life lesson to his students - to confront and navigate through society head on and not reject it as he once did. Rio Nakamura presents their teacher a small cake to mark the event Aguri declared to be his birthday (the day when she and Korosensei first met each other). Having not eaten any sweets for nearly a week Korosensei starts drooling on the cake but nonetheless feels happiness. Before he could blow out the candle however, the cake was destroyed by Shiro, accompanied by Korosensei's former apprentice (No. 2), who is now a hideous tentacled monster.

The two proceed to fight where it was obvious to everyone that No. 2 was able to reach speeds at Mach 40 and Korosensei was barely able to defend himself. Furthermore, Shiro injected himself with a tentacle seed to become superhuman while retaining his intelligence to support the apprentice. When Korosensei began to dodge their attacks more effectively, Shiro ordered No. 2 to attack the defenceless class. As his natural duty, Korosensei took the full brunt of the apprentice's attacks to protect them and he declared that protecting them is not his critical weakness but rather the happiest gift he had been given. Korosensei continues wearing himself down to protect them until Kayano fires a shot at No. 2 to buy her teacher time to heal himself. Despite Korosensei's protests, Kayano was able to damage one of No. 2's tentacles but was pierced right in the chest, the same manner as her sister. Seeing all this, Korosensei turns black as a sign of his rage. Shiro injects the second Reaper with a needle, supposedly a stimulant of some sort, in order to match his power more easily. As Reaper 2.0's attack hurtles towards his 'heart', Korosensei stops it with white energy, much to Shiro's surprise. Fusing all his emotions and colors together, he uses his ranged energy explosion tactic, blasting away both the Reaper 2.0 and Shiro.

While Shiro humiliatingly disintegrated by the barrier, Korosensei knifed his apprentice in the chest. The two share one last exchange, where Korosensei acknowledges his former student's wish and promises to start over should they meet "on the other side", before the Reaper also disintegrated. Korosensei returns to the students, and starts treating Kayano's wounds, having collected her blood prior to the droplets hitting the ground even while fighting his opponents. As the students celebrate Kayano's miracle recovery, Korosensei stares up at the night sky wondering whether he had successfully fulfilled his promise to Aguri.

Final moments
Korosensei then collapsed out of fatigue from his fight with Reaper 2.0 and after noticing the laser's light intensifying, he announced that now was the time for the class to kill him. Once the class reluctantly decided to pin him down, Korosensei tells his students that he left them each a book of advice as a farewell gift and that he would announce one last roll call. As he calls out the list of names, he reminisces on how he is proud of his students who gave meaning to his life.

Nagisa, who chose to deliver the killing blow, becomes overwhelmed by his emotions. In response, Korosensei softly places a tentacle on his cheek and tells that he wants Nagisa to kill him with a smile on his face. After giving each other their final farewells, Korosensei is stabbed in the heart, and his body dissipated into numerous particles of light.

Powers

 * Antimatter physiology: Due to the human experiments of producing antimatter in a living body by Shiro, Korosensei's entire appearance is an octopus-like creature with multiple tendrils that function as his tentacles. Anything aside from Anti-Sensei Weapons will be ineffective in harming him. It is initially believed that he would self-destruct by the time of Class E's graduation; however, it has since been proven that the chances are minuscule at best.
 * Protective coloring: Korosensei can change his skin into any color, utilized for expressing emotions as well as disguise and camouflage.
 * Mucus: The standard trait of an antimatter organism, the mucus that Korosensei has varied uses for support purposes, such as waterproofing, cleansing, adhesive and the replacement of tears.
 * Liquefaction: After he drank the 4th poison made by Manami Okuda, Korosensei developed an ability to control his fluidity. He can morph into a mass of mercury-like slime. In cost of his massive body and tentacles, Korosensei can fit through any space while retaining most of his abilities.
 * Absolute Defense: Another trump card never used until the Okinawa trip, Korosensei can shrink his body into a sphere and the remaining energy from the transformation hardens as a crystallized second layer over his spherical body, making him impervious to harm. After twenty four hours, the external shell liquefies and he will slowly absorb energy over a period of time until he fully regenerates. The only drawback to this in the span of a day, he is rendered immobile and is susceptible to being carried around. The only mechanism that could potentially break through this form is the Spear of Heaven.
 * Energy laser: An ability developed from the Absolute Defense. Korosensei can channel a portion of his inner energy to his tentacles in order to discharge a laser blast.
 * Molting: A once-a-month occurrence, Korosensei can shed his old skin and use it as a protective membrane either on himself or another individual, protecting them from being hurt even from military-grade explosives. Afterwards, his reaction speed may temporarily be reduced, but he can still be fast enough to dodge attacks from the class.
 * Regeneration: If one or more of his tentacles are maimed, Korosensei can regenerate the severed limb. However, this weakens his stamina.
 * Surgery: After he was unable to save Aguri from her fatal injury, he began learning about surgery methods, which he first demonstrated on Azusa to remove her brain tumor without damaging her. He later uses his knowledge to heal and revive Kayano from receiving the same injury as Aguri.
 * Superhuman speed: Korosensei can travel up to the speed of Mach 20. His normally soft and jelly-like head hardens under immense pressure as a process known as the dilatancy phenomenon, which presumably lowers his wind resistance, allowing him to reach inhuman speeds. The only known objects to surpass this are the reusable space shuttle and the transformed Reaper.
 * Superhuman reflexes: Korosensei can react and dodge multiple fires of Anti-Sensei bullets and move away from any harm. His reflexes also allow him to react fast enough to catch or deflect any flying projectile in mid-air using what he has on hand.
 * Speed replication: Korosensei's amazing speed allows him to seemingly copy himself dozens of times using after-imagery. Not only can each individual after-image maintain a task for extended periods of time, but Korosensei's "speed copies" are all capable of taking on widely-different tasks both individually and simultaneously without confusing themselves. This technique was primarily used by Korosensei to teach all of Class 3-E one-on-one when a difficult test approached, allowing Korosensei to work with each student separately to work on their problem subjects.
 * Superhuman senses: Korosensei's sensory perceptions are paramount, and can detect even the most minuscule of changes in temperature, light, sound, and smell. Despite having no visible ears or nose, Korosensei has proven to be able to hear things no normal human can pick up on, as well as detect the most minor of aromas, which aided him greatly in evading the assassination attempts of Class 3-E. Korosensei is especially talented in identifying people by their smell, as well as certain materials (such as explosives and poisons). His eyes are also quite keen, and can keep track of objects going thousands of miles per hour without difficulty, as well as detect the slightest of movements that occur quicker than the blink of an eye.
 * Superhuman digestion: Korosensei can digest anything aside Anti-Sensei weapons, as his teeth can crush any hard materials. Whether he eats something or it enters his body, he can either molten the food first then eject it, or it will just be digested. Although it is not guaranteed that Korosensei will receive no irregularities from what he has eaten, it usually has no tactical disadvantages if it is not a weakness for him.
 * Invisibility: Korosensei can turn invisible, but can still be noticed because his clothes don't turn invisible.

Abilities

 * Genius Intellect: Even before Korosensei gained his powers, he possesses a gifted mind which allows him to learn and understand complex knowledge in a matter of minutes. Since gaining his speed, Korosensei has become hypercognitive, and can process information thousands of times faster than the average human being. With his immense intelligence, he was able to single-handedly teach all the subjects and produce impressive results from Class 3-E. His incredible mentality combined with his speed also allow him to multitask on up to dozens of jobs at a time (teaching the entirety of Class 3-E individually and then some), all of which seemingly being done simultaneously. His greatest feats appear to be coming up with inventions leaps and bounds above contemporary science in a matter of days, such as Ritsu's upgrades and the chemical that granted him Liquefaction.
 * Assassination: As the original Reaper, Korosensei was considered the world's best assassin prior to being betrayed by his disciple. As a result, Korosensei can combat nearly every assassination attempt utilized by both Class E and professional assassins, showing to be extremely observant and stealthy at planning and execution.
 * Eidetic Memory: Korosensei is able to store all that he has learned and use them to his advantage, which led to him being the strongest assassin in the world prior to his capture. In order to become a competent educator to Class 3-E, he has memorized all of the text books in Japan so as to be able to coach his students where their weaknesses lie.
 * Genius Chemist: He was able to create a chemical granted him his Liquefaction ability.
 * Mechanics: Korosensei was able to upgrade Ritsu, designing and implementing approximately 985 modifications and upgrades within less than a single day. He also upgraded her AI to grant her some form of autonomy and sentience.

Weaknesses
Because of his answer to the tentacles when they questioned him from within, Korosensei gained a number of weaknesses when he became the teacher of Class 3-E. Below is a list of such weaknesses recorded by Nagisa Shiota.
 * 1) When he tries to act cool, his weaknesses show.
 * 2) He is surprisingly quick to panic.
 * 3) He is petty.
 * 4) His punches are weak.
 * 5) Boobs (natural weakness).
 * 6) He is humble and subservient to his superiors at work.
 * 7) He has no patience for wire puzzles.
 * 8) He gets motion sickness.
 * 9) He can't sleep on a pillow other than his own.
 * 10) He worries about appearances.
 * 11) He can't handle hot food.
 * 12) Getting wet (natural weakness).
 * 13) Juicy gossip.
 * 14) Hackneyed plot developments make him cry.
 * 15) He gets taken in by rumors.
 * 16) Time right after molting (natural weakness).
 * 17) Time right after regeneration (natural weakness).
 * 18) His body goes rigid when bathed in a special beam of light (natural weakness).
 * 19) When he returns to normal after a serious situation, he gets embarrassed.
 * 20) He can only draw simple pictures.
 * 21) Heat fatigue.
 * 22) He is strict about poolside manner.
 * 23) He can't swim (natural weakness).
 * 24) He likes to gossip.
 * 25) He is weak against the occult.
 * 26) If he can't gather people, he feels worthless.
 * 27) Tone-deafness.
 * 28) He sucks at hiding.
 * 29) He gets carried away by official positions.
 * 30) His heart (natural weakness).
 * 31) Not related, but acts like a doting parent.
 * 32) Gets insurance to avoid getting hurt.
 * 33) He doesn't really know what "your own football" means.
 * 34) He sucks at reporting.
 * 35) He suffers from road rage.
 * 36) Restructuring (natural weakness).
 * 37) Grabbing all his tentacles at once makes him unable to move (Natural weakness).
 * 38) Unintentionally adding insult to injury.
 * 39) Completely oblivious to the mood in moments of high tension.
 * 40) Class 3-E students: Arguably Korosensei's greatest weakness. Although students of 3-E class are subtly aware of this, only three students ever made actual attempts to exploit this factor: Karma Akabane, Nagisa Shiota (using himself as bait), and Ryoma Terasaka (using Nagisa as a suicide bomber the first time, second when he was unknowingly manipulated by Shiro into nearly drowning his classmates, but has redeemed himself since). Exploiting this weakness means one has no qualms of discarding lives of his/her classmates, and even their own; Yanagisawa has demonstrated these qualities.

Foibles
Korosensei hides certain things from his students in his life, which was revealed during the extra chapters. Here are the foibles that Korosensei has for his life as an adult.
 * 1) Painfully narcissistic designs.
 * 2) His room resembles a dumpster.
 * 3) He only makes petty wagers.
 * 4) A bit of a degenerate octopus.
 * 5) His purpose has shifted from using porn magazines to collecting them all, at this point.
 * 6) Starts a nights' off with cassis and orange.
 * 7) Takes forever to decide on an appetizer.
 * 8) Really can't hold his liquor.
 * 9) Kind of like an old geezer.

Class 3-E
In general, Korosensei has a good relationship with all his students, helping by doing anything as he can. As a result, he is highly respected by them. His positive relationship with the class stems from his promise to their former teacher Aguri Yukimura to give them courage and not let any of the students fall behind.

Korosensei does not show favoritism with any student, but is particularly interested in Nagisa Shiota, for not just being quiet and well-mannered but for his talents as an assassin, who eventually followed his path to become a teacher.

Aguri Yukimura
Korosensei's first impressions of Aguri were neutral, seeing her only as a means of escape but commented on her bizarre fashion sense and scatterbrain behavior.

However, as time passed their relationship became close after Aguri taught him what "compassion was". Eventually, Korosensei finally told her of his life, and Aguri expressed her desire to instill courage in her depressed students.

Aguri's death inspired Korosensei to take up teaching, leading to current events with Class 3-E.

The Reaper
His apprentice whom he met killing his father. After witnessed the killing, the child became enamored with it and requested that the Reaper train him as his apprentice. Eager for the assassination opportunities that an apprentice could give, the Reaper took in the boy and trained him.

Even though he taught him everything he knew all while still instilling respect and fear in him and letting him know the difference in power between them, the first Reaper still saw him as his tool rather than a real student, which eventually caused him his apprentice to and abandoned him at a prison after realizing his teacher's view of him.

After the Reaper was captured, he admitted to Aguri that he was dumbfounded on why his student betrayed him, to which she respond that he might wanted to his master to acknowledge him. Many years later, the second Reaper's wish for his master to acknowledge him still lingered even when he was given tentacles by Shiro and right before his death.

Gakuhō Asano
Initially, Asano presented himself as Korosensei's work superior and a major adversary at teaching. He made it a point to stand as a wall between Korosensei and his goals for improving Class E. However, Korosensei managed to overcome those blocks. It was revealed that they share many similarities in how they want to raise strong and capable students and how they are both "monsters" that dedicate themselves to teaching.

After beating Asano, their relationship had noticeably improved; the chairman no longer stood as a barrier to Class E's growth, but rather another assassin aiming to kill Korosensei. During Valentine's Day, Asano offered to allow Korosensei to continue teaching at Kunugigaoka, something Korosensei views as a sign of great honor and respect. On that day, Korosensei had a friendly conversation with Asano, and thanked the chairman for making him truly enjoy his experience as a teacher.

Their shared desire to teach ultimately results in a lot of trust between them. In the final volume, Asano is described as Korosensei's "greatest supporter" and when Korosensei reflects on which assassination attempts were the closest to succeeding, he mentions that he wasn't too worried about participating in Asano's game because he knew the explosion wouldn't be so strong as to injure the other students.

Tadaomi Karasuma
Most of the time, Karasuma seems annoyed and exasperated by Korosensei.Various students remark that, under different circumstances, they could have become close friends. It was noted that Karasuma never called Koro-sensei by his name, addressing him as "octopus". Only during Korosensei's final moments was the only instance where he used his given name.

Irina Jelavić
Most of the time, Irina tries to attack Korosensei, who easily evades her attacks. Even though Korosensei constantly mocks her attempts to kill him as well as her love life, they have a friendly relationship.

Quotes

 * (His vow to Aguri after her death) "So long as I have time... I will use this as a teacher. The students you watch over, I will watch them myself. No matter what, these tentacles will not let them go."
 * (To Nagisa Shiota) "The fact that I'm targeted by everyone is proof that I'm powerful, after all."
 * (To Itona Horibe) "For sensei, assassination is education. The sensei of the assassination classroom becomes stronger as he teaches."
 * (To his apprentice) "My dear student... may you graduate... in peace."
 * (To Gakuho on Valentine's Day) "You didn't let emotions or background cloud your judgement... and evaluate me solely on my aptitude as a teacher. Such a proposal is the happiest report card you could have given me."
 * (To Class 3-E after telling his past) "If I'm going to be killed, it can't be at the hands of anyone else. I want to be killed by you."

Trivia

 * Korosensei has never actually known his real name.
 * Korosensei is usually represented by an octopus due to his appearance of his head and tentacles. Aware of this, he often draws octopus mascots when scoring exam marks and makes jokes about octopuses.
 * He also has a nature similar to an octopus, as he is highly intelligent for his survival skills.
 * His ability to regenerate his tentacles, healing others, bestowing wisdom, and appearance is reminiscent of the akkorokamui.
 * In the popularity poll in Volume 12, Korosensei was ranked 3rd.
 * Korosensei is ambidextrous, being able to write/perform a task with either hands, and being able to work with both hands at the same time.
 * According to Aguri, Korosensei is not Japanese.
 * His necktie is a present from Aguri Yukimura --though, the crescent moon motif is his own design.
 * He once took on the guise of a university professor.
 * Korosensei is portrayed by Jun Fukuyama, who previously portrayed another manipulative protagonist, Lelouch vi Britannia, from the Code Geass series.