Makoto Kobayashi

The main character of the anime (novel). A soul who died and got another chance to re-enter the circle of reincarnation. He enters the body of a boy called Kobayashi Makoto who committed suicide. Now he has to find out what his own sin was and why Makoto died in a limited time or he is sent back.

It's later revealed that he's the soul of Makoto himself, so his sin was to commit suicide.