The school's lady killer, Koga has slept with most the girls in the school and was a bully to all. The only person he never insulted or hurt was Honoka, who he made his conquest to one day get into bed, however she did not feel anything toward him. He was abusive and the exact opposite of Tamotsu, making them bitter enemies and rivals.
He and his brother and did the charm and ended up in Heavenly Host, He showed his cowardness by abandoning his best friend to die, leaving his younger brother when he twisted his ankle because he was "slowing him down" and then he raped and murdered Hatsumi. This lead to his death at the hands of Tamotsu, who tortured him by cutting up his insides with a knife and removing his left eye. He then proceded to stab him repeatedly and dislocate his arm from his shoulder and leaving him to die alone.