Kaigaku

Kaigaku is a former Demon Slayer and Zenitsu Agatsuma's senior disciple under former Hashira 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. After becoming a Demon, he gained black, flame-like markings on his face, fangs, pale skin, and pointed ears. His eyes received the mark of Upper Moon Six.

Kaigaku was usually seen wearing a black kimono over his Demon Hunter uniform with yellow markings.

Personality
Kaigaku was very prideful and self-serving, seeing morality as something based on whether people agreed with him ("right") or not ("wrong"). He valued his own life over all others, easily abandoning the Demon Slayers when he came 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 saw himself as Jigoro's rightful heir and resented Jigoro's decision to make both him and Zenitsu his successors. However, this only served to highlight Kaigaku's impatience and hubris; his inability to use the First Form, the basis of all the others Forms, meant his technique in the advanced Forms could only go so far. This proved to be his undoing, as while Kaigaku 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. He was one of the orphans Gyomei Himejima cared for at his temple. After he was caught by the other children for stealing temple money, he was chased out at night and encountered a Demon, which he then let in to the temple in exchange for having his life spared.

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 was taken in by Jigoro along with Zenitsu, learning the Breath of Thunder. He became determined to become Jigoro's successor and the new Thunder Hashira. 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 peach at him for daring to call Jigoro "grandpa."

Dimensional Infinity Fortress Arc
During the Hashira Training Arc, Zenitsu receives word that Kaigaku had become a demon, and that Jigoro committed seppuku as atonement. Kaigaku had fought against Kokushibo and, upon witnessing the latter's immense power, kneeled before him and begged to become a Demon. As a Demon, Kaigaku ate many people and became powerful enough to become the new Upper Moon Six of the Twelve Demon Moons.

Inside the Dimensional Infinity Fortress, he and Zenitsu seek each other out. Kaigaku is ultimately beheaded by Zenitsu's Breath of Thunder, Seventh Style: Flaming Thunder God. Kaigaku mockingly comments that their master did favor Zenitsu more for teaching him that style, until Zenitsu admits that Flaming Thunder God was his own making in order to fight alongside Kaigaku as equals. Kaigaku notes that Zenitsu was going to die from severe exhaustion, only for him to be saved by Yushiro in the last seconds. Ultimately, Kaigaku‘s head screams in anger as he disintegrates.

Demon Abilities
Enhanced Breath Techniques: As a Demon, Kaigaku's Breath of Thunder techniques were 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 had he been given another year, Kaigaku's strength would have crushed Zenitsu on the spot.

Blood Demon Arts (血けっ鬼き術じゅつ): Kaigaku had completely merged his Blood Demon Arts with his Breath of Thunder sword attacks, successfully creating an effect in which his slashes and sword swings immediately cracked his opponents' skin and burned their flesh, greatly increasing the lethality of his sword strikes. This also gave his attacks a black, lightning-like appearance.

Swordsmanship
Breath of Thunder Style (の): Taught to Kaigaku by a Hashira, Jigoro Kuwajima. But, he was unable to learn the first style despite learning the other styles of the Breath of Thunder. Like Kokushibo, a former Demon Slayer, he could combine his Blood Demon Technique with his Breath Style to increase its power.
 * Second Form: Rice Spirit (ノ) - The swordsman is able to generate five straight attacks around the user in a single moment.
 * Third Form: Thunder Swarm (ノ) - Generates a spinning wave of lightning in all directions.
 * Fourth Form: Distant Thunder (ノ) - The user releases multiple ranged strikes towards a target.
 * Fifth Form: Heat Lightning (ノ) - A single, focused, slashing attack.
 * Sixth Form: Rumble and Flash (ノ) - A series of strong ranged attacks released around the user.

Trivia

 * Kaigaku is the second known Demon Slayer to have become a Demon, the first being Kokushibo.
 * Kaigaku and Tengen Uzui are the only Demon Slayers shown to hold their swords behind their backs.
 * Because of the movement pattern of the Breath of Thunder's first style, it can be speculated that one reason he had difficulty mastering it was due to the way he holds his sword.
 * Kaigaku is heavily implied to be one of Gyomei Himejima's old wards, the same one that betrayed his adoptive family and got all but two of them killed by a Demon in exchange for his own life.
 * Kaigaku received a kimono from Jigoro, the same one that he wore, but Kaigaku never even wore it once.
 * Kaigaku's necklace and bracelet is a fractured Taijitu (taoist symbol). Before he transforms into a Demon, the symbol is yellow (good, enlighted), and it turns blue (bad, darkness, depth) after he becomes one. The fractured symbol itself seems to represent his broken and unstable mentality.

Quotes

 * (To Zenitsu Agatsuma) ""Gramps"? Don't speak of him in that familiar way! Sensei is a former Hashira! Someone who earned the highest title in the Demon Slayer Corps! Almost no one ever gets the chance to be trained by a former Hashira! The time Sensei spends training you is all for nothing! You're an eyesore, okay? Get out of my sight! Why are you here? Why do you insist on clinging here?"
 * (To Zenitsu about his sensei) "I don't care! So what? Are you saying I should be sad? That I should repent? I don't care about anyone who doesn't value me! At every moment I always serve those who know my value!"

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