Height: ~300cm (~9'10")

He is a teacher for class 3-E of Kunugigaoka Junior High School and the main antihero of Ansatsu Kyoushitsu. He claims responsibility for blowing up the moon into a permanent crescent shape and plans to destroy the Earth in a year after teaching the students of class 3-E the art of assassination. His name comes from korosenai (殺せない), meaning "cannot be killed", and sensei (先生), meaning teacher. In this way, he is literally called the unkillable teacher.

Later in the story he is revealed to have been original reaper named Ryushi Korogane.