Kaigaku was a former Demon Slayer, your close friend and Zenitsu Agatsuma's senior  disciple under the former Thunder Hashira, Jigoro Kuwajima. However, sometime during Hashira Training, Kaigaku went on a mission and encountered Upper Rank One, Kokushibo, quickly surrendering to him to save his own skin and later becoming a member of the Twelve Kizuki. He is a demon affiliated with the Twelve Kizuki, holding the position of Upper Rank Six, replacing Daki and Gyutaro after their deaths.