Koro-Sensi

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 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, he would be "a little lecherous, smart, klutzy, kind of stingy and stubborn" and he would smile because he is genuinely a kind person. This prediction happens to reflect his personality; which became forefront after he answered the tentacles. When first introduced, Korosensei was an upbeat 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 Shiota, 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 Ryoma 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 of his students, Yukiko Kanzaki and Kaede Kayano, were being held at and single-handedly beat up all of the high schoolers, scaring them away. One of Korosensei's attributes was that he was able to connect to each student individually, being an objective, empathetic, and observant person who will do almost anything to help his students. This was why all the students respected him even though he would 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 Taiga Okajima and Itona Horibe. This characteristic was revealed to be the side effect of the experimentation with anti-matter tentacles which made him unable to hide his true feelings. He is a pervert. He loves large breasts and femboys so Koro-Sensi owns a lot of erotica and pornography magazines filled with large breasted women and femboys.

Appearance: Korosensei is a three-meter tall yellow octopus with a large head, small eyes, and two tentacle arms. He uses his other tentacles to walk and has hidden nostrils that make him appear to have four eyes. Korosensei wears black academic dress with a yellow crescent moon on the tie as a gift from Aguri Yukimura.

Korosensei (殺ころせんせー) acted as the homeroom teacher of Class 3-E of Kunugigaoka Junior High School, and the anti-heroic overarching protagonist 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".

Abilities

1. 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 the Class 3-E students and professional assassins, showing to be extremely observant and stealthy at planning and execution.
2. 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. His greatest feats appear to be coming up with inventions leaps and bounds above contemporary science in a matter of days.
3. Genius Chemist: Korosensei was able to create a chemical granted him his Liquefaction ability.
4. Mechanics and Programing: 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.
5. Teaching Ability: With his immense intelligence, Korosensei was able to single-handedly teach multiple academic subjects and achieve 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, all of which seemingly being done simultaneously (including providing individual teaching to every student of Class 3-E simultaneously, focusing on the subject each of them needs to study most and even changing his headband as he goes).
6. Eidetic Memory: Korosensei is able to store all that he has learned and use them to his advantage, which led to him being the most prolific 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.
7. Surgery: After he was unable to save Aguri Yukimura from her fatal injury, Korosensei 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 Kaede Kayano from receiving the same injury as Aguri.