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.!~
