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|“||Get out of here... You gonna make me repeat myself?, leave my sight. You know why. The way you cry like a baby all day long. Aren't you ashamed of yourself?.. Good for nothing, any effort spent on you is an absolute waste of time. Our teacher is an extraordinary man. [..] Hey you, don't go being all familiar and calling him gramps! He's a former Hashira, someone who earned the highest tile in the Demon Slayer Corps. Hardly anyone ever gets the chance to be trained by a former Hashira. The time he spends training you is all for nothing its infuriating to see! I hate even looking at you so get out! Why are you even here? Why do you insist on clinging on like a patch of mold?!||„|
|~ Kaigaku berating Zenitsu in the past.|
|“||I know what right and wrong is! Those who appreciate my talent is the right, those who don't and denounce me is the wrong!||„|
|~ Kaigaku revealing his values.|
Kaigaku (獪岳，かいがく) is a major antagonist in the Demon Slayer: Kimetsu no Yaiba manga and anime. He was a member of the Twelve Demon Moons, holding the position of Upper Moon Six after the deaths of Daki and Gyutaro.
He was originally a Demon Hunter and member of the Demon Killing Corps as well as Zenitsu Agatsuma's senior student when they were being taught by Jigoro Kuwajima. He later becomes Zenitsu's archenemy after his treachery.
He is voiced by Yoshimasa Hosoya in the Japanese version of the anime and by Alejandro Saab in the English version, the former of which also voiced Wave from Akame Ga Kill! and Reiner Braun from Attack on Titan.
|“||I always side with those who properly acknowledge me, whenever and at any time.||„|
|~ Kaigaku to Zenitsu at their reunion.|
Although diligent, Kaigaku is an arrogant opportunitist who follows strong and despises the weakness, especially when it obstructs something he wants, as seen with his bullying words against Zenitsu.
Deep down, Kaigaku is also a coward, who cowered in fear at Kokushibou and knelt in front of him, willingly accepting Muzan Kibutsuji's blood and became a demon as a result. Kaigaku also shows no remorse for his own treachery and cowardice, even teasing his master, Jigoro Kuwajima, as a worthless weakling just like Zenitsu after discovering the latter committed suicide out of shame.
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.
When he was a child, Kaigaku was one of 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 in to 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.
After acknowleging the Demons' existence, Kaigaku decided to become a Demon Slayer in order to become strong. During his carrer 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 Style in the Breath of Thunder, while Kaigaku had never mastered it even after mastering the other styles 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, Kaigaku went on a solo mission to hunt down Kokushibou, 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 he accepted and became a man-eating savage. After finding out that his student betrayed him and renounced humanity, Kuwajima committed seppuku out of shame.
Dimensional Infinite Fortress Arc
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons and confronted Zenitsu as a rematch he always desired. Zenitsu informed his master's demise to his treachers classmate, accusing Kaigaku for the one to blame, but Kaigaku nonchalently disagreeded with him. He believed that those who praised him was good and those who don't was evil, and he didn't care about his master's death for ignoring him. Disgusted, Zenitsu called Kaigaku as a trash who can never master Breath of Thunder - First Style, and decided to part ways with his former prefect.
Since he was yet to master his Demon Skills fully, 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 revealed that Flaming Thunder God was his own making, in order to fight alongside Kaigaku as equals, had him not turning into a Demon.
Hearing this, Kaigaku watched in shock and anger as Zenitsu was saved by Yushiro. Eventually, Kaigaku's head screams 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 his desire inflate.
- His name Kaigaku (獪岳), 獪 means wily or sly. 岳 means big mountain. Combining those letters together, it is roughly translated as 'a man who deludes himself as bigger than he really is'.
- As his archenemy Zenitsu was only able to use the First Style of Breath of Thunder, other styles of Breath of Thunder were not revealed until Kaigaku used them. But other styles used by Kaigaku are not likely to be their original styles since Kaigaku is now a Demon with an ability to use Blood Demon Art. Therefore other styles used by Kaigaku are likely to have been corrupted.