Demon Slayer: Why was Zenitsu awake in Infinity Castle? Explained

Zenitsu Agatsuma is a character initially dismissed and typecast by fans of Demon Slayer as the comic relief of the series. As one of the main characters and a traveling companion to Tanjiro, he often falters in the face of adversity.

Nonetheless, he reflexively performs the most impressive Thunder Breathing techniques after losing consciousness. However, the Infinity Castle Arc marked a significant shift in his character development, with Zenitsu surprising everyone by staying awake during his battle with Kaigaku.

Zenitsu, who used to pass out when facing demons, chose to be awake because he had finally overcome his self-doubt and was ready to confront Kaigaku on his terms. For him, this was a personal battle he actively pursued, fueled by the desire to avenge his master and confront a fellow disciple who had strayed onto the wrong path.

Disclaimer: This article contains spoilers from the manga. Any opinion expressed here belongs to the author.

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Zenitsu’s resolve to stay awake during his fight with Kaigaku in Demon Slayer explained

Tanjiro checks up on Zenitsu (Image via Ufotable)

Zenitsu is a highly relatable character for those who struggle with anxiety and fear in their lives. In the early arcs of Demon Slayer, he frequently faints in terror during battles, relying on precise muscle memory to execute Thunder Breathing: First Form—Kaminari no Ichi no Kata: Ichi Iga (Thunderclap and Flash).

Up to this point, this is the only Thunder Breathing form he appears to know. He uses it to gain the upper hand in major fights, such as his battle against the Spider Demon (Son) on Mt. Natagumo, and to rescue the victims Daki had imprisoned within her obi during the Entertainment District Arc.

As the story unfolds, each challenge he encounters helps build his self-confidence. He gradually realizes he can trust himself to stand up and fight for his beliefs without relying entirely on unconscious combat. The Infinity Castle Arc marks a significant step forward for Zenitsu as a Demon Slayer; he confronts his fears and fights Kaigaku, his former senior disciple, face-to-face.

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Zenitsu vs. Kaigaku: A battle of opposites

Kaigaku and Zenitsu will fight in Demon Slayer (Image via Ufotable)

Kaigaku and Zenitsu share a complex history as disciples of Jigoro Kuwajima, the former Thunder Hashira. Though both were competitive, they had little in common. Kaigaku was overly ambitious and wanted his master’s recognition as his sole successor, while Zenitsu, feeling inferior to Kaigaku, often acted like a crybaby and secretly admired him.

However, everything changed when Kaigaku met Kokushibo, the Upper-Rank One demon, during a mission. Given the chance to gain more power in exchange for losing his humanity, Kaigaku seized the opportunity and consumed Muzan’s blood, transforming into a demon.

He soon took over the vacant Upper-Rank Six position following Gyutaro’s death. Jigoro, Zenitsu’s master, was heartbroken by Kaigaku’s choice to forsake his humanity, leading him to take his own life out of shame.

In season 4, episode 7 of Demon Slayer, Zenitsu receives a letter from his sparrow, Chuntaro, informing him of his master’s death and Kaigaku’s betrayal, solidifying his resolve to defeat Kaigaku and uphold his master’s teachings.

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The fight in Infinity Castle

Zenitsu using his Thunder Breathing technique (Image via Ufotable)

In chapter 143 of the manga, Zenitsu senses Kaigaku’s presence in the Infinity Castle. The encounter is immediately hostile, with Zenitsu expressing outrage and anger at his former senior’s actions and their devastating consequences. Kaigaku, who is devoid of humanity, shows little concern, driving them to battle it out.

Kaigaku, as a demon, employs overwhelming and fast lightning techniques enhanced by Blood Demon Art. Despite this, Zenitsu remains awake and perseveres, fighting back harder than ever.

Marking a pivotal shift in his character, he proves that he does not need to rely solely on instinct to become a formidable Demon Slayer. To reinforce this, Zenitsu also reveals a brand-new technique: The Thunder Breathing: Seventh Form - Honoikazuchi no Kami (Flaming Thunder God). This technique is Zenitsu’s original creation, showcasing that he has truly surpassed Kaigaku as Jigoro’s disciple.

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Final thoughts

The Infinity Castle, Muzan’s lair (Image via Ufotable)

In simple terms, Zenitsu’s battle with Kaigaku will demonstrate to fans how much he has developed as a character throughout the series. His choice to stay awake while entering the Infinity Castle signifies his transformation from a scared child to someone prepared to defend his beliefs.

The fifth season will likely follow Zenitsu, Tanjiro, Inosuke, and the other Hashira as they arrive at the Infinity Castle. The upcoming trilogy of movies will bring his clash with Kaigaku to life, establishing Zenitsu as one of the strongest Thunder Breathing users.

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