Zenitsu Agatsuma vs Kaigaku

Zenitsu Agatsuma vs Kaigaku was a battle that took place in the Infinity Castle. It focuses on the battle between Zenitsu Agatsuma against Upper Moon Six, Kaigaku.

Prologue
Zenitsu, while walking through the fortress, encounters his old senior, Kaigaku, whom he hears talking from behind a door. He tells the Demon to come on out as he knows it's him. Kaigaku dislikes the lack of respect Zenitsu shows and tells him that it's no way to talk to his senior.

Zenitsu responds that, because he is a Demon, he no longer considers Kaigaku his senior. Kaigaku sneers at Zenitsu that he's shabby and still just a weakling, then trying to provoke him by asking if Zenitsu had become a Hashira or if he could use anything other than Thunder Breathing's first form. Zenitsu calmly retorts that Kaigaku seems thrilled to be the lowest-ranked of the Upper Kizuki, despite the fact that he's simply filling an empty slot.

Intrigued by Zenitsu's insult, Kaigaku laughs and says that he now knows how to talk back. Zenitsu wants to know why Kaigaku became a Demon, since he used to be someone who was passionate about learning Thunder Breathing and becoming the previous Thunder Hashira's Successor. He proceeds to let out his frustration on Kaigaku by telling him he was the cause of Jigoro's, their master's, death, since Kaigaku becoming a Demon was the reason he committed seppuku and died a slow, agonising death on his own. Kaigaku says he believes that only those who know his value are worth caring about and procedes to mock Jigoro, calling him a senile old man. This is met with great disdain from Zenitsu.

The comment Zenitsu makes towards Kaigaku enrages him, and he tells Zenitsu to not lump him in the same boat as him. He draws his blade and initiates the battle.

Battle
Zenitsu, quick as he is, draws his sword and slashes past Kaigaku's neck, calling him trash and saying he was too slow. Zenitsu then is slashed by Kaigaku with the Thunder Breathing: Second Form - Lightning Ball. Zenitsu remarks that Kaigaku has eaten too many people and can no longer tell what is right or wrong. Kaigaku continues to use multiple breathing technique forms and argues with Zenitsu. While falling, Zenitsu reflects on his relationship with Kaigaku before, and reflects how he and Kaigaku could not get along. He jumps onto a wall and uses his own breathing form: Thunder Breathing: Seventh Form - Honikazuchi no Kami. Kaigaku is beheaded while falling, and curses Zenitsu while falling with him.

Aftermath
While falling, Zenitsu is rescued by Yushiro. Yushiro insults Kaigaku that it is not fun to die alone. Kaigaku dies and Zenitsu is rescued.