Zenitsu Agatsuma vs Kaigaku was a battle that took place in the Infinity Castle. It focuses on the battle between Zenitsu Agatsuma against Upper Rank 6, Kaigaku.
Prologue
Zenitsu, while walking through the castle, 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 Ranks, despite the fact that he's simply filling an empty slot.
Zenitsu shaming and reminding Kaigaku that he is only a replacement for the Upper Ranks.
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 and Kaigaku clash.
Kaigaku attempts to kill Zenitsu using his Blood Demon Art enhanced Thunder Breathing: Fourth Form - Distant Thunder, Zenitsu quick as he is, draws his sword and slashes past Kaigaku's neck, calling him trash and saying he was too slow. Kaigaku realizes that Zenitsu is much more versatile and skilled with his swordsmanship than before. However, he still believes that no matter how much Zenitsu trains, he can never defeat an Upper Rank like him. Zenitsu then is slashed by Kaigaku with the Thunder Breathing: Second Form - Rice Spirit. Zenitsu remarks that Kaigaku has eaten too many people and questions him he can no tell what is right or wrong, to which Kaigaku pridefully responds that he does.
Zenitsu defeats Kaigaku with Seventh Form: Honoikazuchi no Kami.
As Kaigaku uses the Thunder Breathing: Third Form - Thunder Swarm, he says that those who judge him correctly and accept him for who he is are correct and those who don't accept him are wrong. Kaigaku then uses the Thunder Breathing: Fifth Form - Heat Lightning and taunts at Zenitsu, praising his own Blood Demon enhanced Forms. He explains that once his Thunder Breathing forms slash Zenitsu's body, his skin and flesh burns and cracks apart as he uses the Thunder Breathing: Sixth Form - Rumble and Flash. 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.
References