Koro-sensei is a teacher for class 3-E of Kunugigaoka Junior High School and the main antihero 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.
Koro-sensei is made up of anti-matter. He will inevitably die in March, whether the students assassinate him or not.
It is later revealed that the chance of this happening was less than 1%.
Koro-sensei was previously known as the "God of Death."
Koro-sensei did not actually destroy the moon, a rat infused with anti-matter did.