In "Demon Slayer", Kaigaku starts out as a student under Zenitsu's master, Jigoro Kuwajima. Kaigaku's story takes a dark turn, however, when he forsakes his life as a Demon Slayer and goes over to the enemy--becoming one of them instead. It is his pursuit for power and his rejection of weakness that spurs this transition.
Being Upper Rank 6 of the Twelve Kizuki, Kaigaku makes for a formidable foe for Zenitsu and is a constant irritation to him both mentally and physically. His character arc serves as a brutal reminder of what happens when power is chosen over human decency.