Korosensei

Korosensei is the homeroom teacher for Class 3-E of Kunugigaoka Junior High School, and the main antihero of Assassination Classroom. He is responsible for gouging out a large chunk of the moon, making it a permanent crescent moon. As if it's not adequate; 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 likes sweets. He 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
Korosensei was born and raised on Earth. At some point before the events of the manga, he was a test subject at a research facility that studied tentacles. It was there that he met and befriended Aguri Yukimura. However, one day there was an explosion and Aguri became fatally injured. Her final wish to Korosensei was to teach her students (Class E), all the while thinking of what a wonderful teacher he will be.

Right before the start of the series, Korosensei destroyed 70% of the moon, forever leaving it in a crescent shape. Korosensei reveals to the world's leaders that he has decided to destroy the Earth as well the following year during March. 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.

Later, Korosensei came with Karasuma, who was from the Ministry of Defense, to the front of Class-E and introduced himself. Karasuma explained the situation and demanded the students to assassinate the new homeroom teacher. As an example, he tried to assassinate Korosensei, but Korosensei used his mach 20 speed to dodge Karasuma's strikes, and even meticulously trimmed his eyebrows. After that, Karasuma revealed that the reward for killing him would be 10 billion yen, and Korosensei simply looked down on them while his skin became covered in green stripes. After Karasuma ended his explanation and showed the weapons, Korosensei declared that they should spend the last year in a meaningful fashion.

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.