Kaigaku is a character who appears as a supporting antagonist during the events of the Infinity Castle. His history is deeply intertwined with several main characters, and his actions serve as a catalyst for one of the story's most personal confrontations.

Before becoming a demon, Kaigaku was a member of the Demon Slayer Corps. He was an orphan who survived a harsh childhood by stealing, a period that brought him under the care of Gyomei Himejima, who ran a temple for orphans. Kaigaku’s time at the temple ended in betrayal; after being caught stealing and chased out, he encountered a demon and, to save his own life, led it back to the temple by extinguishing the protective wisteria incense. This act resulted in the deaths of the other children and caused Gyomei to be wrongfully imprisoned for murder.

Later, Kaigaku became a disciple of Jigoro Kuwajima, the former Thunder Hashira, training alongside Zenitsu Agatsuma. Kaigaku was a skilled and ambitious student, mastering several forms of Thunder Breathing. He was determined to be named Jigoro's sole successor and become the next Thunder Hashira, a goal that made him resentful when Jigoro declared that both he and Zenitsu were his successors. He was openly contemptuous of Zenitsu, viewing him as a cowardly weakling who brought shame to their master.

Despite his skill, Kaigaku's primary motivation is an overwhelming desire for self-preservation and status. His childhood taught him that strength and survival are the only things that matter, and he believes that anyone who validates his worth is right, while those who do not are wrong. This selfish worldview ultimately defines his path. During a mission, he encountered Kokushibo, the Upper Rank One demon. Overwhelmed and unwilling to die, Kaigaku immediately surrendered and chose to become a demon to save himself. He took Muzan Kibutsuji's blood and, after devouring many humans in a short time, rose to become the new Upper Rank Six of the Twelve Kizuki, replacing Daki and Gyutaro.

In the story, Kaigaku serves as a dark mirror to Zenitsu and a testament to Jigoro Kuwajima’s failure. His betrayal has severe consequences; upon learning that his disciple became a demon, Jigoro committed seppuku to atone for Kaigaku's sins. This makes the confrontation between Kaigaku and Zenitsu deeply personal, driven by Zenitsu's need to avenge their master and hold Kaigaku accountable for his actions. Kaigaku, in turn, feels no remorse, even mocking Jigoro’s death and remaining intent on proving his superiority over Zenitsu. His key relationships are therefore defined by betrayal: he betrays Gyomei's care, Jigoro's teachings, and any bond he might have had with Zenitsu.

As a demon, Kaigaku's abilities become formidable. He retains his mastery of Thunder Breathing, utilizing its Second through Sixth Forms. However, as a demon, his Blood Demon Art grants him the power of electrokinesis, allowing him to generate and manipulate black lightning. This enhances his swordsmanship, as his slashes now crack and burn the flesh of his opponents. His weapon is a katana made from his own flesh that mimics a Nichirin sword. Despite his power and status, he is still unable to master the First Form of Thunder Breathing, Thunderclap and Flash, a weakness that highlights the gap in his foundational skills compared to Zenitsu. His development concludes in a duel with Zenitsu within the Infinity Castle. There, Zenitsu unveils a newly developed Seventh Form of Thunder Breathing, called Honoikazuchi no Kami or God of Fire and Thunder, which allows him to finally surpass and behead his former senior. Even in death, Kaigaku remains unrepentant, screaming in denial and anger as he perishes, a figure whose entire existence was defined by his selfish refusal to value anything above his own life.