Korosensei

"Nurufufufu~"

- Korosensei's trademark laugh

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

Prior to being a teacher, he was the master assassin known as the 1st "God of Death ".

Appearance
Korosensei had the appearance of an yellow bipedal octopus. He was about three meters tall, and possessed a large, bulbous head with a large smile and small, beady eyes. He used two feelers as arms, with two fingers each, and utilized the rest of his tentacles to walk with. He wore a black academic dress, a small black square academic cap with a yellow tassel, and a large black tie with a yellow crescent-shaped moon on it. According to Korosensei himself, his nostrils are so small yet when discovered, it seemed that he had four eyes.

The color of Korosensei's face changed depending on his mood, and is as follows:
 * Yellow/Green = Happy, Emotionless/Neutral
 * Yellow and Green Stripes = Mockery
 * Orange with a Red Circle = Correct Answer
 * Purple with a Dark Red X = Incorrect Answer
 * Pink = Sleepy/Smitten
 * Purple = Shock/Panic
 * Blue = Sadness
 * Red = Annoyance
 * Black = Fury
 * Pure White = Poker face OR A mix of every emotion

As a human, he had black hair that reached to his nape and dark eyes. His height was of a normal man's. He was notably attractive and was described as having a smile kinder than anyone else. He usually dressed in formal clothing, wearing trench coats whenever on a mission. After being taken prisoner by Yanagisawa, he was put in patients' scrubs until his escape.

Personality
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. Despite this, he cared for his students' 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 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 and scaring them away.

Korosensei was very smart to the point that he could teach almost every subject by himself. He 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. He was also rather kind, and can switch from silly to serious on the fly.

One of Korosensei's attributes was the fact that he was able to connect to each student individually. 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, he 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. In one instance, he brought Nagisa and Karma all the way from Japan to Hawaii to catch a movie because all three of them expressed interest in that movie, all the while using his tentacles to deflect dust and debris from harming the two of them.

According to Aguri Yukimura, 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 a lot stubborn... or something like that." 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.

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 most vile 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 and became the master assassin known as the God of Death. 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.

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 and requested that the God of Death train him as an apprentice. Eager for the assassination opportunities that an apprentice could give, the God of Death took in the boy and trained the boy. He taught him everything he knew all while still enlisting respect and fear in him and letting know the difference in power. After several successful jobs, the apprentice turned on the God of Death as he was attempting to escape a prison facility. The apprentice, now the current God of Death, sought his master's title and abandoned him just to fuel his ego. Betrayed, the God of Death 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 Shiro. Aguri 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 God of Death 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 rat). When the rat did die at the end of its lifespan it caused a volatile reaction with the antimatter in it's 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 God of Death 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 God of Death 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 God of Death 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 lands 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.

Aguri's 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 God of Death vowed to help those children. He left a note then left the facility for a nearby mountain. He 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 striked 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 anihilate 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. Korosensei then asks what they will do before he blows the Earth up, and Nagisa responds that they will kill him. Korosensei gets green stripes on his face, telling them that they can go home if they succeed in killing him now. After that, Kayano says the name she thought of for sensei: Korosensei.

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 outmanoeuvres 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, the result that the class except for Karma dropping in rank.

The students' actions had also attracted the attention of the professional hit man known as the God of Death. 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 God of Death.

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 God of Death, bonding with Aguri during his captivity and promising to teach her students before she died during his escape. He also stated that if no one managed to kill by March, he will 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 onboard 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. He soon dodged Chairman Asano's attack and was offered the choice to continue teaching after March; however Korosensei declined.

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/embarassing 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 "Lance of the Heavens" 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 gen God of Death with a needle, supposedly a stimulant of some sort, in order to match his power more easily. As the second gen God of Death's attack hurtles towards his 'heart', Korosensei stops it with white energy, much to Shiro's surprise. Fusing all his emotions and colours together, he uses his ranged energy explosion tactic, blasting away both the God of Death 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 God of Death 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.

After healing Kayano, Korosensei collapses due to exhaustion and injuries from his battle against Shiro. He addresses Class 3-E that the assassins should not miss their dying target and that the inevitable has come. He further states that all good times must come to an end, hence the classroom. The students, after briefly discussing whether or not to kill Korosensei with their own hands, ultimately all decides to lend their hands on giving him the final peace. Utilizing the weakness #37, students grabbed all of Korosensei's tentacles, sealing his maneuverability, with Nagisa Shiota chosen to deliver the coup de grace to the teacher's heart. Korosensei addresses to his students that because of lacking time, he does not have a time to say his farewell to each and every single students so instead he left things he wants to say to each of his students in advice books in their classroom. With less than half an hour away until Lance of the Heavens fires its gun, Korosensei takes the attendance of his beloved Class 3-E pupils one final time.

Korosensei, Assassinated
Before Korosensei calls his pupils their attendance, he bids farewell to his fellow faculty staff Tadaomi Karasuma and Irina Jelavić, with Karasuma making his promise to coach the future of Class 3-E students. After completing his final attendance, Korosensei expresses his joy to be assassinated by his beloved pupils. Nagisa Shiota, trembling with uncertainty, resolves to kill Korosensei by 'killing his emotion', but Korosensei stops Nagisa's unstable brainwave and directs him to kill 'calm and smile'. Regaining his composure, Nagisa tearfully bids his teacher good bye and plunged an anti-Korosensei knife into the heart of the teacher through his necktie. With the killing blow to the heart, Korosensei smiles as he turns into shining particles of light, then disintegrating into nothingness, leaving only the clothes and graduation cap that Koro-sensei always wore every day.

Korosensei was assassinated one minute before midnight and before Lance of Heaven fires again, the whole of Class 3-E wailed in grief to see their beloved teacher depart for eternity after being assassinated.

His dearest wishes has finally come true and he also able to find his answer by becoming a teacher of Class 3-E that he always seek ever since Aguri Yukimura gave her last wish before her demise, hoping that he will find the answer as he teaches her class which makes his wish came true by witnessing the student that she was entrusted to him has been guided to the light from the dark while Koro-sensei himself was able to find his answer by having a peaceful death through assassination of Class 3-E, killed not by the darkness of the past but by the light that Class 3-E gave to give him peace for eternity.

Time After Time - A Lasting Legacy
After the assassination has been confirmed by the government, the students went inside the classroom to see a large book of advice on each of their desks. Even though their beloved teacher and target is gone, his presence still lingers in the hearts and minds of his students and will continue to do so in times to come.

The next day, Class 3-E went to the graduation ceremony of Kunugigaoka Junior High School; no longer did the academy's main body personnel looks down on students of E class. It is revealed that Korosensei also managed to meet up and have a chat with Nagisa's father a month before his demise, asking him to continue supporting and blessing his son for the road ahead. Nagisa could only cry at the fact Korosensei, even in death, continues to look after him. Except for Karma Akabane who chose to remain at Kunugigaoka Academy, the other 27 students departed from junior high school, venturing towards an uncertain future.

The government rewarded the class with the 30 billion yen as promised. However the majority was handed back, using only some of it to purchase the mountain classroom, make donations and cover the bare minimum of education and living expenses. Seven years later some of the former classmates reunited to refurbish the building. None of them had considered the necessity to build a gravestone for both Korosensei and Aguri, believing that doing so will reinforce the fact that they are both truly gone. His final legacy, however, was a student he considered his favourite: Nagisa becoming an educator himself, using a fraction of his tact for assassination to coerce his new rowdy and rebellious into submission.

Powers and Abilities
Antimatter Physiology: Due to the human experiments of bonding antimatter to the living body by Shiro, Korosensei's entire appearance is an octopus-like creature with multiple tendrils that are his tentacles. It is initially believed that he will self-destruct by the time of Class E's graduation; however it has since been proven that the chances are minuscule at best
 * 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 slow temporarily but he is still fast enough to dodge the assassins of 3-E.
 * Regeneration: If one or more of his tentacles are maimed, Korosensei can regenerate the severed limb like a lizard regenerating its tail. However, this leaves him weakened as his reaction speed is dampened, albeit not enough to topple the teacher.
 * 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 might be able to penetrate and smash this sphere is Lance of the Heavens.

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. With his immense intelligence, he was able to single-handedly teach all the subjects and produce amazing results from the bottom of the barrel class of 3-E.
 * 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 object to surpass this is a space shuttle which is used for space launch which reaches the speed of Mach 23.
 * Superhuman Reflexes: Korosensei, with his speed, 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 such as handkerchief or even a chalk. Another example of this is when facing off against Asano Gakuho, he is able to open a test book, solve a denoted question on a small slip of paper and close the book without the explosive contained in the book detonating. Plus, he is also fast enough to molt and toss his protection skin over the principal when a normal grenade detonated point-blank in his face.
 * Assassination: As the original God of Death, Korosensei was considered to the world's best assassin prior to being betrayed by his disciple. As a result, Korosensei can combat nearly every assassination utilized by both Class E and professional assassins.
 * Eidetic memory: Korosensei is deduced to have an eidetic memory, as he 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 an educator to class 3E, he has memorized all of the test books in Japan so as to be able to coach his students where their weaknesses lie.

Weaknesses
Because of his answer to the antimatter tentacles when they questioned him from within, here are the weaknesses Korosensei gained when he became the teacher to class 3E.
 * 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 weaknesses)
 * 6) He is humble and subservient to his superiors
 * 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
 * 13) Juicy gossip
 * 14) Hackneyed plot developments make him cry
 * 15) He gets taken in by rumors
 * 16) Time right after molting
 * 17) Time right after regeneration
 * 18) His body goes rigid when bathed in a special beam of light
 * 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's strict about poolside manner
 * 23) He can't swim
 * 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) Gets carried away by official positions
 * 30) Heart
 * 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
 * 37) Grabbing all his tentacles at once makes him unable to move
 * 38) Unintentionally adding insult to injury
 * 39) Completely Oblivious to the mood in moments of High tension.
 * 40) Class 3-E students: Undeniably 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 themselves as baits), and Ryoma Terasaka (using Nagisa as a suicide bomber the first time, second 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.

Class 3-E
In general, he has a good relationship with all his students and is highly respected by them. Korosensei does not show favouritism 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. 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.

Gakuhō Asano
Initially, the Chairman presented himself as Korosensei's superior and a major adversary. 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. In the end, Korosensei performed "maintenance" on the chairman and as of Chapter 126, their relationship is noticeably better with the chairman no longer standing as a barrier to Class E's growth, but another assassin aiming to kill Korosensei. During valentine's day, the Chairman offers to allow Korosensei to continue teaching at Kunigaokoa, something Korosensei views as a sign of great honour and respect. On that day, it was the last time Koro-sensei met the Chairman and having a friendly chat, thanking him for making Koro-sensei truly enjoy his experience as 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 behaviour. As time passed however their relationship became close to the point where Korosensei told her of his life and Aguri telling him of her desire to install courage in her depressed students. Aguri's death later inspires Korosensei to take up teaching, leading to the current events with Class 3-E. It is noted that Korosensei's perverted personality originated after seeing Aguri's breasts thus beginning his fixation for busts and pornography.

Quotes

 * (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 tentacles about what he wanted to be) "I want... to be weak."
 * (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"
 * (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, theses tentacles will not let them go"

Trivia

 * His motto is "Learning and killing are of one body."
 * Korosensei appears as a playable character in the video game J-Stars Victory Vs.
 * He was called "Stupid Chicken Pervert Octopus" during class 3-E training using code names, rather than the code name he suggested, 'the imperial prince of fate of the eternal wind'.
 * In the popularity poll in Volume 12, he was ranked 3rd.
 * He once disguised as university professor (in chapter 136)
 * When his tentacles asked him what he wanted, Korosensei's response was: "I want... to be weak".
 * His necktie is a present from Aguri Yukimura . Though, the crescent moon motif is his own design.
 * According to Aguri, Korosensei is not Japanese.
 * His weaknesses for boobs was already seen when Aguri shows him her ogre themed thumb top which Aguri remark him as a pervert even before tentacle ask him for what he want to be.
 * He is killed by Nagisa Shiota