Koro-sensei is a teacher for class 3-E of Kunugigaoka Junior High School and the main protagonist of Ansatsu Kyoushitsu. He was responsible for carving out a large chunk of the moon to become a permanent crescent moon and plans to destroy the earth after teaching class 3-E for a year. His name comes from Korosenai (殺せない) means "cannot be killed" and sensei, means teacher. Literally, his name means unkillable teacher.

As a young boy, Denji inherits his father's debts from the Yakuza. After meeting Pochita, he becomes a Devil Hunter for the Yakuza in an attempt to clear his debt. After Denji was betrayed and murdered by the Yakuza, which were under the control of a Devil, Pochita brought Denji back to life by becoming his heart. He now has the ability to become a devil and human hybrid by pulling the cord on his chest. In his chainsaw man form, Denji is equipped with a chainsaw for a head and one chainsaw blade coming out of each wrist. After meeting Makima, he became a Public Safety Devil Hunter. (Source: Chainsaw Man Wiki)