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“	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!	”
	— Kaigaku to Zenitsu Agatsuma about his Sensei, Chapter 144

Kaigaku (獪 (かい) 岳 (がく) , Kaigaku^?) is a former Demon Slayer and Zenitsu Agatsuma's senior disciple under former Thunder Hashira Jigoro Kuwajima. However, he becomes a Demon to save his own life and later became the Upper Rank Six (上 (じょう) 弦 (げん) の陸 (ろく) , Jōgen no Roku^?) of the Twelve Kizuki, replacing Daki and Gyutaro after their deaths.

Appearance

As a human, Kaigaku is a fit young man with black hair and turquoise eyes. After becoming a Demon, he gains four black, flame-like markings on both sides of his cheeks, fangs, pale skin, and pointed ears. His eyes receive the kanji of Upper Rank (上 (じょう) 弦 (げん) , Jōgen^?) and Six (陸 (ろく) , Roku^?).

Kaigaku is usually seen wearing a black kimono with a blue obi over his Demon Slayer uniform with yellow markings. He also wears multiple necklaces with blue magatama-like pendants that were yellow when he was human.

Personality

Kaigaku is very prideful and self-serving, seeing morality as something based on whether people agree 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 and begging him to let him become a Demon.

Because he easily learned to use the Second through Sixth Forms of the Thunder Breathing, 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 Forms, means his technique in the advanced Forms could only go so far. This proves to be his undoing, as, while Kaigaku can only augment his abilities after becoming a Demon, Zenitsu is 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.^[1]

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 Thunder Breathing. 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.

Synopsis

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."^[2]

Infinity Castle Arc

During the Hashira Training Arc, Zenitsu receives word that Kaigaku had become a Demon, and that Jigoro committed seppuku as atonement. Kaigaku had the unfortunate luck of encountering Kokushibo and, upon witnessing the latter's immense power, kneeled before him and begged to become a Demon. As a Demon, Kaigaku had eaten many people and became powerful enough to become the new Upper Rank 6 of the Twelve Kizuki.

Inside the Infinity Castle, he and Zenitsu seek each other out. Kaigaku is ultimately beheaded by Zenitsu's Thunder Breathing: Seventh Form - Honoikazuchi no Kami. Kaigaku mockingly comments that their master did favor Zenitsu more for teaching him that form, until Zenitsu admits that the technique 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. Kaigaku's head screams in anger as he disintegrates.

Abilities and Powers

Demon Abilities

• Enhanced Breathing Techniques: As a Demon, Kaigaku's Thunder Breathing techniques are 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 or possibly even less, Kaigaku's strength would have crushed Zenitsu on the spot.

• Supernatural Physical Prowess: As an Upper Rank Demon, Kaigaku's physical capabilities have been augmented to supernatural levels due to the high concentration of Muzan's blood present in him. The base level speed of an Upper Rank can reach subsonic, proven when Upper Rank 3, Akaza, was moving so fast that even two extremely talented swordsman, Tanjiro Kamado and Inosuke Hashibira could only perceive sparks and dust clouds from his blinding movements, the same goes for their immense reflexes and reaction time. The base level physical strength of an Upper Rank is superhuman as well, proven when the weakest Upper Rank, Daki, could punch a hole into a building with her fists. The durability of an Upper Rank's skin and flesh is way stronger than Nichirin, proven when the neck of the second weakest Lower Rank, Rui, couldn't be cut by Tanjiro Kamado. Giyu Tomioka also noted that all Upper Rank Demons have limitless stamina and never suffer from fatigue.

• Enhanced Mental Prowess: Like all Upper Rank Demons, who harbor extremely high concentration of Muzan Kibutsuji's blood, Kaigaku possesses superhuman mental capabilities. He could process information and think clearly at supernatural speeds, proven by fighting a Hashira-level Demon Slayer, who can think at equally or slightly slower speeds. As another testament to his mental capabilities, he has incredible concentration abilities and wits.

Blood Demon Art (血 (けっ) 鬼 (き) 術 (じゅつ) , Kekkijutsu^?):

• Electricity Manipulation: To complement his prowess with the Thunder Breathing, Kaigaku has the ability to manipulate electricity and infuse it with his swordsmanship to enhance the power of his attacks and to make up for his inability to perform its simplest technique: First Form: Thunderclap and Flash. Furthermore, his electricity has been demonically enhanced, creating a sword effect that cracks his opponent's flesh and burns it, greatly augmenting the lethality of his attacks. This also turns the electricity he manipulates a deep shade of black.

Swordsmanship

Thunder Breathing (雷 (かみなり) の呼 (こ) 吸 (きゅう) , Kaminari no kokyū^?): Kaigaku was taught by a former Hashira, Jigoro Kuwajima in the usage of this Breathing Style and immediately stuck out as a natural prodigy due to having mastered it easily. But despite his natural talent, Kaigaku was was unable to learn the first form of the Breathing Style despite learning the other styles of the Thunder Breathing. Since he can't perform the first form, he uses electricity to make up for that weakness. Therefore, his techniques are more powerful compared to when a normal swordsman uses it.

Battles

Trivia

• Kaigaku is the second known Demon Slayer to have become a Demon, the first being Kokushibo.
• Because of the movement pattern of the Thunder Breathing's first form, it can be speculated that one reason he had difficulty mastering it was because he placed his scabbard/sheath on his back instead of either of his waists.
• Kaigaku received a kimono from Jigoro, identical to the one he himself wore, but Kaigaku never wore it once.^[1]
• Kaigaku's necklace and bracelets are Taijitu/magatama (tomoe) symbols. 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 magatama itself seems to represent his broken and unstable mentality.
• Kaigaku ranked 21st place in 2nd character popularity poll with 865 votes.

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?"^[8]
• (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!"^[9]

References