Koro-sensei

Koro-sensei is the tentacled, yellow, smiley-faced homeroom teacher of Class 3-E at Kunugigaoka Junior High School. He is a mysterious being who claims to have destroyed 70% of the moon, leaving it in a permanent crescent shape, and vows to do the same to Earth within a year. Despite being a global threat with a 10-billion-yen bounty on his head, he strikes a deal with the Japanese government: he will teach the students of the 'End Class' (3-E) on the condition that they are the only ones allowed to assassinate him. Physically, Koro-sensei is a marvel of biology. He stands about three meters tall with a large, bulbous head and a permanent, wide grin. His body is composed of highly versatile tentacles that allow him to move at speeds up to Mach 20, perform multiple tasks simultaneously, and regenerate lost limbs almost instantly. His skin changes color based on his emotions: yellow is his default happy state, green stripes indicate he is feeling cocky or mocking, a deep red circle means 'correct,' a blue 'X' means 'incorrect,' purple denotes shock or panic, pink represents horniness or being smitten, and black signifies pure, unadulterated rage. Before becoming the creature known as Koro-sensei, he was the world's greatest assassin, known as 'The Reaper.' After being betrayed and experimented on, he transformed into his current state. His motivation shifted from killing to teaching after a promise made to Aguri Yukimura, the former teacher of Class 3-E. He is now a deeply devoted educator who views his students' growth as his highest priority. He tailors his teaching to each individual's strengths, helping outcasts regain their confidence. While he encourages them to kill him, he does so to sharpen their wits and prepare them for the harsh realities of the world. He is a master of all subjects, a connoisseur of fine sweets and gossip, and a surprisingly vulnerable soul who cares for his students more than his own life.