Now that Upper Moon Six siblings, Daki and Gyutaro, have been defeated in the Entertainment District arc, the question who holds the ranking has been much speculated by Demon Slayer anime-only fans. Since Muzan’s Twelve Kizuki system follows a power structure, the next in line will be Kaigaku, who will make his appearance in the Infinity Castle arc. However, Kaigaku is a character who has connections to a few important characters in the series, the first being Zenitsu Agatsuma, then Rock Hashira Gyomei Himejima, and Kokushibo, among others.
Of the three primary names mentioned above, he holds no blood relations to them, but he does have social connections to them. He and Zenitsu were trained under the same teacher. With Gyomei, Kaigaku used to be one of the orphan kids that the Rock Hashira looked after. Kokushibo, on the other hand, turned him into a demon after Kaigaku willingly submitted himself to the Upper Moon One. Although Kaigaku is one of the supporting protagonists in the Infinity Castle arc, his relevance becomes stark when pitted against Zenitsu. But why is that?
Note: This article contains spoilers from the Demon Slayer manga.
In Demon Slayer, Zenitsu and Kaigaku used to train together under the former Thunder Hashira
Jigoro Kuwajima, a former Thunder Hashira of the Demon Slayer Corps, was a significant figure in Zenitsu and Kaigaku’s life. He was the one who brought the two together by making them his pupils. While there were other students who eventually abandoned his training to be worthless, Zenitsu and Kaigaku stuck by.
However, during the Hashira Training arc, Zenitsu receives a letter from Chuntaro that states Kaigaku becoming a demon after encountering Kokushibo, who offered him his blood. After Jigoro hears that Kaigaku, one of his two successors of being the Thunder Hashiras, has gone astray from the respectable path of demon slaying to turning into a demon himself, Jigoro commits seppuku (ritualistic suicide). This news affects Zenitsu greatly since he looks at Jigoro as a father figure who had believed in him despite his cowardice and meek nature. Interestingly, this point was a major character development incident for Zenitsu, who shows a more serious and mature side in the narrative.
As Zenitsu progresses through his life and training beside Tanjiro to defeat Muzan and his followers, Kaigaku banks on the opportunity to strengthen himself by consuming several strong people to attain the ranking of Upper Moon Six. The fateful meeting of the two students of Jigoro Kuwajima finally happens inside the Infinity Castle. Here, Zenitsu, who had always been seen fighting demons in his sleeping state, remains awake during the whole time in his fight against the new Upper Moon Six.
Kaigaku had often considered Zenitsu to be inferior to him in terms of strength and swordsmanship, which changes when the Demon Slayer outclasses the Upper Moon Six and decapitates him with his brilliant speed before the demon could even react. While Zenitsu initially shows reluctance to kill Kaigaku since they share a history, Kaigaku takes the opportunity to land several fatal blows on him. However, that soon changes when Zenitsu beheads the demon with his new technique, Honoikazuchi no Kami. This also shows that Zenitsu had the strength and ability to kill Kaigaku from the beginning, but he had held himself back. But as is the case with demons, even after their head gets separated from their body as a result of beheading, they retain their consciousness and slowly disintegrate. This was earlier seen with Daki and Gyutaro, and the same happened with Kaogaku as well.
Upon encountering Zenitsu’s new Thunder Breathing move, the defeated Upper Moon Six angrily comments how the Demon Slayer was easily favored by Jigoro despite lacking the talent. However, Zenitsu shuts off the claim by admitting that he made the technique on his own to fight on equal grounds with Kaigaku, who had mastered all forms of Thunder Breathing except for the First Form. In a petty move, Kaigaku observes how Zenitsu will eventually die as a result of severe exhaustion, but his hopes are crushed when Yushiro saves Zenitsu at the last moment, mocking him in the process by calling out on his selfishness and cowardice. What follows is Kaigaku’s frustrated screams as he disintegrates.
Thus, the relationship between Zenitsu and Kaigaku culminated with the death of the demon. Kaigaku was a despicable figure, whose personal motivation in life was to survive, even if it meant at the cost of jeopardizing others’ lives. The earliest instance of this was seen when he led a demon to the temple where a pre-Hashira Gyomei looked after the orphans, among which Kaigaku was one.
On the other hand, Zenitsu’s presence in the series was often seen as a comic relief until it became serious. Although considered a cry-baby initially, the fandom acknowledged his strength, which peaked against his fight with Upper Moon Six. Later on, he was able to hold off against Demon King Muzan despite being worn out by the effects of Anti-Kibutsuji Drug. And by the end of the series, he did eventually gain the strength of a Hashira even though he wasn’t one himself.
Suffice to say, if not for anything else, Kaigaku’s presence was instrumental in giving Zenitsu the greatest character development in the series. Although ironic, as Kaigaku died, the cheer and appreciation for the Thunder Breathing Demon Slayer only heightened.