Korosensei was the homeroom teacher of Class 3-E of Kunugigaoka Junior High School, and the secondary protagonist and 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". Korosensei has the appearance of a yellow 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 seemingly jelly-like limbs - 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 can 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, 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. 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, 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 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, 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 of his students, Yukiko and Kaede, 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, empathic, and observant person who will do almost anything to help his students. This was why all the students respect 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 Okajima. This characteristic was revealed to be the side effect of the experimentation which made him unable to hide his true feelings.