Kaigaku remembers his unfortunate encounter with Upper Rank One, Kokushibo. He comments it isn't shameful to kneel before an overwhelmingly powerful foe, recalling when he quickly surrendered to the demon. He believes as long as he is alive, he can always do something, even if he has to scrape his head on the ground, drink dirty water or steal to survive. Kokushibo offers Kaigaku a portion of Muzan's blood from him if he wants to become more powerful, and if Muzan accepts him, he can take a place with among the Upper Ranks. Kokushibo advises it takes time to become a powerful demon, as powerful as a strong swordsman, and at that he gives Kaigaku a large amount of blood since he is also a breath user. He tells Kaigaku to be thankful for the blood he is given but threatens to behead him should he spill a drop.
At the time, his whole body was terrified before his transformation, now facing Zenitsu, Kaigaku's fear is non-existent. He is assured no matter how powerful he is, Zenitsu can never beat an Upper Rank. Declaring everybody has to die eventually and saying Jigoro and Zenitsu are no exception, Kaigaku uses the Thunder Breathing, Second Form: Rice Spirit to kill him. Zenitsu takes notice of his five simultaneous attacks and deducing he must have eaten numerous people already. Asking if the demon knows right from wrong, Kaigaku responds that those who accept him are "right" and those who don't are "wrong" as he uses the Thunder Breathing, Third Form: Thunder Swarm. He then uses the Thunder Breathing, Fifth Form: Heat Lightning, hitting Zenitsu and burning him. As he asks how his former swordsman how he likes his new-found demon enhanced forms, Kaigaku follows with Thunder Breathing, Sixth Form: Rumble and Flash, hitting him with numerous slashes.
As Zenitsu falls after being hit, he remembers back to his days training alongside Kaigaku. Ironically, while Zenitsu only manage to do the first form, Kaigaku was able to do all but the first of Thunder Breathing. Zenitsu knew then Kaigaku was never gonna be a Hashira. One day, Zenitsu had hit a senior demon slayer and Kaigaku reprimanded then mocked him for doing so. He knew Kaigaku virulently hated him, Zenitsu himself also hated Kaigaku greatly. But despite this, he genuinely respected Kaigaku for his work ethic and drive, and despite his place as special within Jigoro's students, it never satisfied him. Finally, he considers Kaigaku's feelings like a box of happiness in his heart; his happiness always leaked through and he must close the box to feel satisfied.
Apologizing to Jigoro and Kaigaku, he lands on the wall and stops his fall. As a confused Kaigaku looks on, Zenitsu uses the Thunder Breathing, Seventh Form: Honoikazuchi no Kami and speeds upwards, decapitating Kaigaku. As the Upper Rank demon becomes enraged he never saw it coming, he curses his former master for teaching Zenitsu something he never knew and playing favourites. Zenitsu replies Jigoro was never such a person; he created this form for the day he would fight alongside him.