Kaigaku was a former Demon Slayer and Zenitsu Agatsuma's senior disciple under the former Thunder Hashira (鳴なり柱ばしら, Nari Bashira?), Jigoro Kuwajima. However, sometime during Hashira Training, Kaigaku went on a mission and encountered Upper Rank One, Kokushibo, quickly surrendering to him to save...
