Kaigaku was a former Demon Slayer who later betrayed the Demon Slayer Corps and became a demon, eventually rising to the position of Upper Rank Six of the Twelve Kizuki, sharing the title with Daki and Gyutaro before replacing them after their deaths. He was once a senior pupil of Jigoro Kuwajima, the former Thunder Hashira, and trained alongside Zenitsu Agatsuma, with whom he shared a bitter rivalry.
Kaigaku is emblematic of the dark path of ambition, a character who sought strength at any cost and ultimately lost his humanity in the process.
Appearance[]
As a human, Kaigaku had a tall and lean frame with medium-length black hair styled with an upward part. He had sharp eyes and typically wore the standard Demon Slayer uniform.
After becoming a demon, his appearance changed dramatically. His skin turned a grayish-blue, with black markings running along his body like cracks. His hair grew longer and spikier, and he gained the demon’s characteristic slit pupils. His mouth became more angular, and a kanji for "Upper Six" (上弦の陸) was etched into his left eye, signifying his rank among the Twelve Kizuki.
Personality[]
Kaigaku was arrogant, prideful, and deeply ambitious. He believed in power above all else, looking down on anyone weaker than himself and hating being disrespected or belittled. His ego was fragile, and he resented being overshadowed — particularly by Zenitsu, whom he viewed as an incompetent fool unworthy of sharing the same mentor.
While Kaigaku shared Zenitsu’s training under Jigoro, he despised their master’s ideals of discipline and humility. Unlike Zenitsu, Kaigaku refused to learn all six forms of Thunder Breathing and only mastered the first five, believing that to be sufficient.
Ultimately, Kaigaku chose to become a demon rather than die at the hands of Kokushibo. He willingly accepted demonhood in exchange for power, showing his cowardice and betrayal of the Demon Slayer Corps.
Story[]
Background[]
Kaigaku was once a starving orphan taken in by Gyomei Himejima and raised in a temple with other children. However, Kaigaku revealed himself to be selfish and survival-oriented, going as far as sacrificing others to demons to save himself. His betrayal led to the deaths of everyone in the temple, with only Gyomei surviving — an act that foreshadowed Kaigaku’s future as a traitor.
Later, he was trained by Jigoro Kuwajima, the Thunder Hashira, and became one of his successors. However, Kaigaku's pride led to disdain for his mentor's values and envy toward Zenitsu, who, despite his apparent cowardice, was chosen to inherit Jigoro’s legacy due to mastering the First Form with exceptional power.
Kaigaku's failure to become the Thunder Hashira, combined with his inner hatred, pushed him further into darkness.
Betrayal and Demon Transformation[]
During a mission, Kaigaku encountered Kokushibo, the Upper Rank One demon. Realizing he couldn’t win, Kaigaku chose to grovel and submit, allowing Kokushibo to turn him into a demon. This betrayal devastated Jigoro, who later committed seppuku out of shame for his student’s actions.
After transforming, Kaigaku's abilities were enhanced exponentially with demonic power, and he was granted the position of Upper Rank Six, replacing Gyutaro and Daki.
Final Battle: Zenitsu vs. Kaigaku[]
Kaigaku’s final appearance occurs in the Infinity Castle Arc, where he faces Zenitsu in a long-awaited one-on-one duel. Kaigaku is smug and taunting, believing himself far superior due to his rank and demon powers.
However, Zenitsu reveals a new secret technique: the Seventh Form: Honoikazuchi no Kami (Flaming Thunder God), a form of Thunder Breathing that he created himself, representing his growth beyond Kaigaku’s shadow.
Zenitsu defeats Kaigaku in a spectacular final move, beheading him. In his dying moments, Kaigaku expresses disbelief and bitterness, unable to accept that Zenitsu surpassed him. Zenitsu mourns not for Kaigaku’s life, but for their former master, who died heartbroken due to Kaigaku’s betrayal.
Powers and Abilities[]
Thunder Breathing (雷の呼吸, Kaminari no Kokyū)[]
Kaigaku was trained in Thunder Breathing, the same sword style used by Zenitsu. However, he only mastered Forms One through Five, refusing to learn the Sixth Form, and lacked the humility to fully embody the style’s spirit.
- First Form: Thunderclap and Flash (雷の呼吸 壱ノ型 霹靂一閃): He could perform this classic dash strike, though with less finesse than Zenitsu.
- Other Forms (Second to Fifth): These involved a mix of horizontal slashes, multi-hit combinations, and fast-paced blitz strikes. His demonic strength amplified their destructive power.
Electrified Blood Demon Art[]
After becoming a demon, Kaigaku combined Thunder Breathing with a Blood Demon Art that let him generate and control black lightning. This electrified his slashes, allowing his attacks to burn, paralyze, and tear apart opponents with shocking force.
- Electrified Thunder Techniques: Kaigaku’s hybrid techniques fused swordsmanship and demonic energy, allowing for strikes that tore through terrain and defenses.
- Lightning Aura: He radiated a dangerous electrical field in combat, capable of burning opponents just by proximity.
Regeneration and Demon Physiology[]
As an Upper Rank demon, Kaigaku had advanced regeneration, strength, and speed far beyond that of ordinary demons. He could recover from most wounds instantly, though he was still vulnerable to beheading by a Nichirin Blade.