Kaigaku

Kaigaku (獪岳 Kaigaku) is a minor antagonist in the manga/anime series Demon Slayer: Kimetsu no Yaiba. As a human, he was a Demon Slayer and Zenitsu Agatsuma's fellow classmate under the former Thunder Hashira Jigoro Kuwajima. After transforming into a demon, he became a member of the Twelve Kizuki, holding the position of Upper Rank Six following the deaths of Daki and Gyutaro.

At some point after he joined the Demon Slayer Corps, he was defeated by Kokushibo while on a mission and quickly surrendered to save his own skin, which led him to become a demon and later rise to the Upper Ranks of the Twelve Kizuki. His betrayal would lead Kuwajima to commit seppuku out of his remorse for his pupil's crimes and the Demon Slayer Corps to brand Kaigaku as a traitor, in addition to sending his former classmate, Zenitsu, into a rage.

Ironically, despite being able to grow much stronger as a demon and to become a Upper Rank, Kaigaku would ultimately pay the price for his treachery and cowardice when he was finally slain by Zenitsu during the Infinity Castle Arc.

He is voiced by Yoshimasa Hosoya (who also voiced Wave in Akame Ga Kill! and Reiner Braun in Attack on Titan) in Japanese and by Alejandro Saab (who also voiced Trebol in One Piece, Derrick Arden in Black Butler, Michael Gedōin in Blue Exorcist, Kaiju No. 9 in Kaiju No. 8 and Atsushi Kanamoto in Sword Art Online) in English. Although diligent and talented, Kaigaku was an arrogant opportunist with an inflated sense of self-preservation. Like many orphans living on the streets, he grew up resorting to thievery and other immoral means to survive. Yet unlike other orphans who joined the Demon Slayer Corps, he never developed a desire to protect people or slay demons.

Hypocritically, Kaigaku claims to follow the strong and despise weakness. He bullied and belittled Zenitsu for his lack of dignity or perseverence. Yet deep down, Kaigaku was also a coward, who cowered in fear at Kokushibo and knelt in front of him, willingly accepting Muzan Kibutsuji's blood and becoming a demon as a result, and did what Zenitsu himself would not do, sell out his mentor and fellow pupils to the demons. Additionally, unlike other demons, he retained his memories after his transformation, showing that his evil nature was as genuine as it came.

Kaigaku had also shown no remorse for his own treachery and cowardice, even insulting his master, Jigoro Kuwajima, as a worthless weakling just like Zenitsu after discovering the latter committed suicide out of shame. Furthermore, even after having been defeated by Zenitsu and on the brink of death, he felt no regret for any of his actions, only feeling excitement in the hope that Zenitsu would die alongside him. After Zenitsu was saved by Yushiro, Kaigaku screamed in rage as his body disintegrated into ash. Early life Kaigaku was an orphan and a thief, who had to eat scraps and steal to survive. This established a certain belief in his head, he started to believe that strength was the only thing worth respecting, and those who can‘t see true strength or are too weak aren’t worth respect.

First Treachery When he was a child, Kaigaku was one of the orphans Gyomei Himejima cared for at his temple, and had rely on theft and eating scrap to survive. However, he had developed a self-serving view upon survival. After he was caught by the other children for stealing temple money, he was chased out at night. After that, Kaigaku encountered a Demon, which he then let into the temple in exchange for having his life spared, in order to seek revenge against Gyomei and other children. It eventually caused Gyomei to be wronged and sent into prison, until he had been rescued and joined the Demon Slayer Corps.

Second Treachery After acknowledging the Demons' existence, Kaigaku decided to become a Demon Slayer in order to become strong. During his career as a Demon Slayer, Kaigaku was taken in by Jigoro Kuwajima, the Thunder Pillar, along with Zenitsu, and taught the Breath of Thunder style. Kaigaku became determined to become Jigoro Kuwajima’s successor and the new Thunder Pillar. However, Kuwajima revealed that Zenitsu would become his successor, since Zenitsu had potential to master the First Form of Thunder Breathing, while Kaigaku had never mastered it even after mastering the other Forms from the Second to the Sixth.

Believing he was losing his master's favor, Kaigaku began to despise Zenitsu for being merely a weak and timid man who earned his master's favor out of dumb luck. Thus, he started to bully Zenitsu constantly.

Later on, Kaigaku went on a solo mission to hunt down Kokushibo, the Upper Moon One, but was overpowered. After the defeat, Kaigaku kneeled before Kokushibo and begged for his life. Kokushibo offered Muzan's blood to turn Kaigaku into a demon, which Kaikagu accepted, causing his transformation into a People-eating savage. After finding out that his student betrayed him and renounced humanity, Kuwajima committed seppuku out of shame. After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons due to his power and confronted Zenitsu as a rematch he always desired. Zenitsu revealed his master's demise to his teacher's classmate, accusing Kaigaku for the one to blame, but Kaigaku nonchalantly disagreed with his former classmate, believing that those who praise him were good and those who don't were evil. To make matters worse, Kaigaku didn't care about his master's death for ignoring him. Disgusted, Zenitsu called Kaigaku a piece of trash who can never master Thunder Breathing - First Form, and decided to part ways with his former prefect.

Since Kaigaku had yet to master his Demon Skills fully, Zenitsu ultimately won the duel with his newest technique, Thunder Breathing: Seventh Form: Flaming Thunder God. Kaigaku scornfully commented that their master does favor Zenitsu more for teaching him that style, until Zenitsu revealed that Flaming Thunder God was his own making, in order to fight alongside Kaigaku as equals.

Hearing this, Kaigaku watched in shock and anger as his old classmate was saved by Yushiro. Eventually, Kaigaku's head screamed in anger as he disintegrated. Yushiro scornfully declared that Kaigaku will be given nothing since he never learned how to give, and Kaigaku's die-alone fate was sealed once he decided to let Blood Demon Art (血鬼術 Kekkijutsu):

Electricity Manipulation: Complementing his breathing and combat style, Thunder Breathing, Kaigaku can use his Blood Demon Art to generate and manipulate electricity. Kaigaku 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 gash 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. To make matters worse, the cut made by Kaigaku's Blood Demon Art expands and spreads onto other body parts. Had Zenitsu not having the fortune to encounter Yushiro who had medical knowledge in time, the former would have died from his wounds. Thunder Breathing (雷の呼吸 Kaminari no kokyū): Kaigaku was taught by a former Hashira, Jigorō 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 regardless of his expertise of learning the other styles of the 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. Interestingly, Inadama is also his surname (revealed in Kimetsu Academy) Third Form: Thunder Swarm (参ノ型 聚蚊成雷 San no kata: Shūbun Seirai) - Kaigaku surrounds his target 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, effectively turning his sword into a makeshift gun. 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 target. 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 powerful ranged electricity attacks that strike his opponent from multiple directions.