Kaigaku

Kaigaku was a Demon Slayer in the Demon Slaying Corps, and Zenitsu Agatsuma's senior disciple under Jigoro Kuwajima. However, he became a demon to save his own life and later became the Upper Moon Six of the Twelve Demon Moons, replacing Daki and Gyutaro.

Appearance
As a human Kaigaku was a fit young man with black hair and turquoise eyes. As a demon he gained black, flame-like markings on his face, fangs, and pale skin. As an upper moon, he gained kanji on his eyes.

He usually wears a black kimono over his demon hunter uniform, that has yellow markings on it.

Personality
Kaigaku is very prideful and self-serving, seeing morality as something based on whether people agreed with him ("right") or not ("wrong"). He values his own life over all others, easily abandoning the Demon Slayers when he comes face-to-face with Kokushibo's strength and begging him to make him a demon.

Because he easily learned to use the Second through Sixth Forms of the Breath of Thunder, he sees himself as Jigoro's rightful heir and resents Jigoro's decision to make both him and Zenitsu his successors. However, this only serves to highlight Kaigaku's impatience and hubris; his inability to use the First Form, the basis of all the others, meant his technique in the advanced Forms could only go so far. This proves to be his undoing, as while he could only augment his abilities after becoming a demon, Zenitsu was able to develop a completely new Seventh Form through dedicated training.

History
Kaigaku was an orphan and a thief, who had to eat scraps and steal to survive. It is heavily implied that he was an orphan Gyomei Himejima cared for: the one who disobeyed his rules, went out at night, was caught by a demon, then had his life spared by sacrificing the others.

Kaigaku developed the belief that strength and survival were the only things worth respecting, and was willing to grovel and beg for his life if it was what it took to survive. Eventually, he taken in by Jigoro Kuwajima along with Zenitsu, and learned the Breath of Thunder style. He became determined to become Jigoro's successor and the new Thunder Pillar. Jigoro, however, insisted that both he and Zenitsu would become his successors, despite Zenitsu's weakness and timid nature.

Natagumo Mountain Arc
Although Kaigaku isn't seen in-person, Zenitsu remembers him in a flashback, where Kaigaku threw a rock at him for daring to call Jigoro "grandpa".

Dimensional Infinity Fortress Arc
During the Pillar Training Arc, Zenitsu received word that Kaigaku had become a demon and, out of shame, Jigoro committed seppuku to atone. Kaigaku had fought against Kokushibo and, upon witnessing the other's immense power, kneeled before him and begged to become a demon. As a demon, Kaigaku ate a multitude of people and became abnormally powerful, enough so he filled Daki and Gyutaro's vacant position as Upper Moon Six of the Twelve Demon Moons.

Inside the Dimensional Infinity Fortress, he and Zenitsu seek each other out. Kaigaku was ultimately beheaded by Zenitsu's Breath of Thunder Seventh Style : Flaming Thunder God. Kaigaku mockingly commented that their master do favor Zenitsu more for teaching him that style, until Zenitsu admitted that Flaming Thunder God was his own making, in order to fight alongside Kaigaku as equals, had him not turning into a Demon. Kaigaku notes that Zenitsu was going to die from severe exhaustion only to be saved by Yushiro in the last seconds. Ultimately, Kaigaku head screams in anger as he disintegrated.

Demon Abilities
Enhanced Breath Techniques: As a demon, Kaigaku's Breath of Thunder techniques have been reinforced and increased in strength. However, he was still being attuned to his newfound demon abilities at the time of his death. Yushiro commented that given another year, Kaigaku's strength would have crushed Zenitsu on the spot.

Swordsmanship
Breath of Thunder Style (の): Taught to Kaigaku by a former Pillar, Jigoro Kuwajima. Despite being unable to master the first style, Kaigaku was capable of learning the other five styles of the Breath of Thunder.
 * Second Style: Lightning Ball (ノ) - A series of five straight attacks all performed in a single moment.
 * Third Style: Swarming Mosquito Thunder (ノ) - A spinning wave attack in all directions.
 * Fourth Style: Distant Thunder (ノ) - A ranged strike at a distance.
 * Fifth Style: Heat Lightning (ノ) - A slashing attack that cracks skin and burns flesh.
 * Sixth Style: Electric Lightning Shower (ノ) - A series of strikes that crack the target's body.

Trivia
Kaigaku is the second known Demon Slayer to have become a Demon, the first being Kokushibo.

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