Koro-sensei

Korosensei is the homeroom teacher of Class 3-E at Kunugigaoka Junior High School and the central figure of the series. A yellow, tentacled creature capable of moving faster than Mach 20, he destroyed most of the moon a year before the story begins, leaving it permanently shaped like a crescent. The Japanese government promises a fortune to whoever kills him before graduation, a task handed to the outcast students of Class 3-E, including Nagisa Shiota and Karma Akabane.

Before his transformation, he was a legendary contract killer known by the epithet the Reaper, raised in poverty and taught that only skill, money, and death could be trusted. Meeting the teacher Aguri Yukimura changed him completely, giving him his first real experience of compassion despite his monstrous new power.

Beneath his goofy, dessert-loving exterior, Korosensei is a remarkably attentive mentor who studies each student closely, tailoring lessons that push them toward both the perfect assassination and genuine personal growth.