Kaigaku

 A young demon who attained the position of Upper Moon Six after the deaths of Daki and Gyutaro. Originally one of the orphans under Gyōmei Himejima, he was chased out by the other children for stealing temple money and encountered a demon, which he then let into the temple in exchange for having his life spared, thus leading to their deaths and Gyōmei being falsely arrested. He then becomes a member of the Demon Slayer Corps, learning the Thunder Breathing from Jigoro along with Zenitsu, before betraying the Slayer Corps and allowing himself to be transformed into a demon by Kokushibo, which leads Jigoro to commit seppuku in atonement. Unlike Zenitsu, who only mastered the first of the six techniques of the Thunder Style, Kaigaku only mastered the other five. He is killed when Zenitsu strikes him with a seventh, exclusive technique developed by him.