Until you die, you haven't lost. You may rub your face in the dirt... you may lose your home or drink muddy water... people may curse you for stealing money... but as long as you live on... you can win someday. You will win. I have moved forward with that belief.
— Kaigaku's ideology in Box of Happiness
Kaigaku was a former Demon Slayer and Zenitsu Agatsuma's senior disciple under the former Thunder Hashira, Jigoro Kuwajima. However, sometime during the Hashira Training Arc, Kaigaku encountered and lost to Upper Rank 1, Kokushibo, and surrendered to him to save his own life, thus becoming a Demon and later a member of the Twelve Kizuki.
As a human, Kaigaku is a muscular young man of average height 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 also receive the kanji of Upper Rank (
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.
Though outwardly brash and arrogant, Kaigaku is in fact cowardly and selfish. Like many orphans living on the streets, he grew up resorting to thievery and other immoral means to survive. This has caused him to develop an intense form of self preservation, often doing what in his own best interest at the expense of others. This is best illustrated in his interactions with Gyomei Himejima's group of orphans.
At some point after this, Kaigaku met and became the disciple of Jigoro Kuwajima. Because of his great talent with a sword, he was poised to become the next Thunder Hashira (
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 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.
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 Kuwajima "grandpa" while continuing to berate him for still trying to learn Thunder Breathing.
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.
Immense Speed & Reflexes: As a user of Thunder Breathing and an Upper Rank Demon, Kaigaku's movements speed and reflexes are at a superhuman level. He can keep up and overwhelm Zenitsu Agatsuma, a user of the same Breathing Style as well as one of the fastest humans in existence for the majority of the fight.
Electrokinesis: Complementing his Breathing Style, Thunder Breathing, Kaigaku's Blood Demon Art allows him to generate and manipulate lightning and electricity. He mainly infuses electricity 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.
Master Swordsman: Kaigaku is a Hashira-level swordsman with his skill in Thunder Breathing, albeit not as skilled as the current Taisho-Era Hashira, he was able to beat Zenitsu Agatsuma, a Hashira-level swordsman who previously fought Upper Ranks, in swordsmanship for the majority of his bout with him.
Thunder Breathing (
- Second Form: Rice Spirit (
弐ノ 型 稲 魂Ni no kata: Inadama?) - Kaigaku unleashes five electrified arching slashes all in a single moment.
- Third Form: Thunder Swarm (
参ノ 型 聚 蚊 成 雷San no kata: Shūbun Seirai?) - Kaigaku surrounds the enemy with powerful waves of electricity and sword slashes that slice up the enemy in all directions.
- Fourth Form: Distant Thunder (
肆ノ 型 遠 雷Shi no kata: Enrai?) - Kaigaku generates powerful lightning strikes in several directions with his blade from a far distance.
- Fifth Form: Heat Lightning (
伍ノ 型 熱 界 雷Go no kata: Netsu Kairai?) - Kaigaku unleashes a powerful ranged electrified sword slash upwards that burns the flesh of his opponent.
- Sixth Form: Rumble and Flash (
陸ノ 型 電 轟 雷 轟Roku no kata: Dengō Raigō?) - One of the most powerful Thunder Breathing techniques. Kaigaku releases a series of strong ranged electricity attacks that strike his opponent from multiple directions.
Flesh Nichirin Katana: Like his senior peer, Kokushibo, Kaigaku harbors a fleshy and demonically modified version of a standard Nichirin Katana he used when he was a Demon Slayer. Unlike Kokushibo's katana, however, his katana is nearly identical to his original one, having a black lightning pattern on both sides of his yellow tinted blade along with a black and gold handle and finally a sword sheathe of the same color on his back. It can also be assumed that his flesh blade facilitates the use of his Blood Demon Art as he can channel his blood through it when he unleashes his techniques.
- 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 Kuwajima, his sensei, identical to the one he himself wore, but Kaigaku never wore it once.
- Kaigaku's necklace and bracelets are Taijitu/magatama (tomoe) symbols. Before he transforms into a Demon, the symbol is yellow (good, enlightened), 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.
- (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!"
- (To himself) "There is no shame in kneeling before someone in overwhelming strength. As long as you stay alive, it will work out somehow. Until you die, you haven't lost. You may rub your face in the dirt... you may lose your home or drink muddy water... people may curse you for stealing money... but as long as you live on... you can win someday. You will win. I have moved forward with that belief."