Kaigaku was a former member of the Demon Slayer Corps. He and Zenitsu Agatsuma, trained by Thunder Hashira Jigoro Kuwajima. He became envious of Zenitsu's cowardly nature and resented him. Upper Moon 1 Kokushibo transformed Kaigaku into a demon. Kaigaku possesses exceptional swordsmanship skills in the Thunder Breathing technique. As a demon, he has enhanced physical abilities. He hates Jigoro for wanting him to share his tittle with Zenitsu and wants to kill Zenitsu, who is a human.