Korosensei

Korosensei is the homeroom teacher for Class 3-E of Kunugigaoka Junior High School, and the main antihero of Assassination Classroom. Korosensei originally claimed to be responsible for creating the permanent crescent moon. As if it's not adequate; he claims that he plans to destroy the earth after "teaching" class 3-E for a year.

Appearance
Korosensei has the appearance of an octopus. He is about three meters tall, and has a large, bulbous head with a large smile and small, beady eyes. He uses two feelers as arms, with two fingers each, and uses the rest of his tentacles to walk with. He wears a black academic dress, a small black square academic cap with a yellow tassel, and a black tie with a yellow crescent-shaped moon on it.

The color of Korosensei's face changes depending on his mood, and is as follows:
 * Yellow/Green = Happy, Emotionless
 * Yellow and Green Stripes = Mockery
 * Orange with a Red Circle = Correct Answer
 * Purple with a Dark Purple X = Incorrect Answer
 * Pink = Sleepy
 * Purple = Shock/Panic
 * Blue = Sadness
 * Red = Anger
 * Black = Extreme Anger

Personality
Korosensei is very smart to the point that he can teach almost every subject by himself. He loves sweets almost extreme cases and is also shown to be a very perverted character, a trait he shares with his student Okajima.

One of Korosensei's attributes is the fact that he is able to connect to each student individually. This is the reason why all the students respect him despite the fact that he is going to blow up the Earth in a year.

History
As a child, Korosensei was born into one of the most vile and corrupt slums. Growing up, he suffered abuse from his family, and watched as corrupt cops ignored the constant number of people who were killed on a daily basis in the slums which he lived in. It was an environment where everyone could easily betray you to the point that you could only trust yourself. Through these hardships, Korosensei immediately developed into a sociopathic killer and began to make a name for himself 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 buisnessman and father. The man's child witnessed the killing. Immediately, he became enamored with the beauty of the kill and requested that Korosensei train him as an apprentice. Eager for the assassination oppurtunities that an apprentice could give, Korosensei immediately and harshly trained the boy. After several successful jobs, the apprentice turned on the God of Death as he was attempting to escape a prison facility. The apprentice, the current God of Death, sought his master's title and abandoned him just to fuel his ego. Betrayed, Korosensei was captured and thrown into a research labratory run by Shiro. Shiro'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. Yukimura was brought in to keep a closer eye on the experiments as Shiro considered her easily expendable if Korosensei attempted to break free and take her hostage. As the tests continued, Korosensei began to slowly bond with her as she worked on compiling exam questions for her class.

However, a problem arose when Shiro and his researchers began to question what would happen to Korosensei's antimatter cells when he died. To answer this question, Shiro's 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 of age, however, 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. Seeing this, Shiro was horrified at the results. His subordinates then began running calculations and discovered that the same thing will happen to Korosensei by March 13 next year. Earth would be destroyed in the same way if left unchecked, prompting Shiro to order his subordinates start preparations to kill Korosensei. A mortified Aguri overhears this and decides to go speak to Korosensei to break the news to him. When she does however, Korosensei reveals that it does not bother him as he expected a death such as this from the start. Telling Aguri to leave, Korosensei finally unleashes his full power that he gained from the experiments to escape as a visibly fuming Shiro watches from a nearby security camera. Korosensei then proceeded to rampage violently, killing off several guards and in the process damaging Shiro's left eye. However, during the escape Aguri is mortally wounded when she activates a special tripmine meant for Korosensei to the latter's horror. Her final wish to Korosensei was to teach her students (Class E), all the while thinking of what a wonderful teacher he will be.

Although not responsible for destroying seventy percent of the Moon, Korosensei claims to have done the deed to the world's leaders, and that he has decided to destroy the Earth as well in March the following year. He then tells the world leaders that he wants 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 Korosensei; however, they also added another condition: he would not harm the students.

Plot
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 that BB bullets, created specially by America for harming him, are harmless to the students, but not to him 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 gets angry, saying that they held no regard of 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.

Weaknesses

 * 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
 * 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) Grabbing all his tentacles at once makes him unable to move

Class 3-E
In general, he has a good relationship with all his students and is highly respected by them.

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 an adversary to him.

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"

Trivia

 * The nickname "Korosensei" means unkillable teacher and is a combination of the words korosenai (殺せない) and sensei (先生). It was given to him by Kaede.
 * 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.
 * In the popularity poll in Volume 12, he was ranked 3rd.