Kaigaku is a supporting antagonist in the manga and anime Demon Slayer: Kimetsu no Yaiba. He was a member of the Twelve Kizuki, holding the position of Upper Moon Six after the deaths of Daki and Gyutaro.
Originally a demon hunter and a member of the Demon Slayer Corps, as well as Zenitsu Agatsuma’s senior pupil when they were being taught by Jigoro Kuwajima. Following his betrayal, Kaigaku later becomes Zenitsu’s archenemy. His cowardice would eventually lead to his downfall as Zenitsu overcomes his insecurities while Kaigaku mocks Jigoro. Kaigaku’s Thunder Breathing training would lead his Blood Demon Arts to be related to lightning.
Personality
Although diligent, Kaigaku is an arrogant opportunist who follows strength and despises weakness, especially when it obstructs something he desires, as seen with his intimidating words towards Zenitsu.
Deep down, Kaigaku is also a coward, who cowered in fear in front of Kokushibou and knelt before him, willingly accepting Muzan Kibutsuji’s blood and becoming a demon as a result. Kaigaku shows no remorse for his own betrayal and cowardice, even mocking his master, Jigoro Kuwajima, as a useless weakling like Zenitsu after learning that the latter committed suicide out of shame.
Biography
Early Life
Kaigaku was an orphan and thief, who had to eat leftovers and steal to survive. This instilled a certain belief in his mind, which led him to believe that strength was the only thing worth respecting, and those who fail to see true strength or are too weak do not deserve respect.
The Demon´s First Betrayal
As a child, Kaigaku was one of the orphans Gyomei Himejima cared for at his temple, depending on theft and eating scraps to survive. However, he had developed a selfish view of survival. After being caught by the other children stealing money from the temple, he was kicked out at night. Afterward, Kaigaku encountered a demon, whom he let into the temple in exchange for sparing his life, in order to get revenge on Gyomei and the other children. This eventually led to Gyomei being wronged and sent to prison, until he was rescued and joined the Demon Slayer Corps.
Second Betrayal
After acknowledging the existence of demons, Kaigaku decided to become a demon hunter to become strong. During his career as a demon hunter, Kaigaku was taken in by Jigoro Kuwajima, the Thunder Pillar, along with Zenitsu, and taught the Thunder Breathing style. Kaigaku was determined to become Jigoro Kuwajima’s successor and the new Thunder Pillar. However, Kuwajima revealed that Zenitsu would become his successor, as Zenitsu had the potential to master the First Form of Thunder Breathing, while Kaigaku had never mastered it even after mastering the other Forms from Second to Sixth.
Believing he was losing favor with his master, Kaigaku began to despise Zenitsu for being just a weak and timid man who gained his master’s favor through sheer luck. Thus, he began to constantly intimidate Zenitsu.
Later, Kaigaku went on a solo mission to hunt down Kokushibou, the Upper Moon One, but was overpowered. After his defeat, Kaigaku knelt 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 savage man-eater. After learning that his pupil had betrayed him and renounced his humanity, Kuwajima committed seppuku out of shame.
Infinity Castle Arc
Following the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Kizuki due to his power and confronted Zenitsu for a rematch he always desired. Zenitsu revealed his master’s death to his teacher’s former classmate, accusing Kaigaku of being the culprit, but Kaigaku indifferently disagreed with his former classmate, believing that those who praised him were good and those who did not were bad. To make matters worse, Kaigaku did not care about his master’s death as he ignored it. Disgusted, Zenitsu called Kaigaku garbage who could never master Thunder Breathing – First Form, and decided to part ways with his former master.
As Kaigaku had not fully mastered his demon abilities, Zenitsu eventually won the duel with his newest technique, Thunder Breathing: Seventh Form: Flaming Thunder God. Kaigaku mockingly commented that his master favored Zenitsu more for teaching him this style, until Zenitsu revealed that Flaming Thunder God was his own creation, in order to fight alongside Kaigaku as equals.
Upon hearing this, Kaigaku watched in shock and anger as his former classmate was saved by Yushiro. Eventually, Kaigaku’s head screamed in rage as he disintegrated. Yushiro contemptuously declared that Kaigaku would receive nothing, as he never learned to give, and Kaigaku’s fate was sealed when he let his desires inflate and dominate him.
Trivia
- Kaigaku may be similar to Pigma Dengar from the Star Fox series due to his betrayal leading to the death of his original superior. However, Kaigaku’s betrayal that led to Jigoro’s death was indirect and confirmed, while Pigma deliberately attempted to make his own superior, James McCloud, die, but may have failed.
- Kaigaku’s name (獪 岳) contains the following symbols: Kai (獪) meaning shrewd or crafty, and Gaku (岳) meaning great mountain. Combining these characters, it roughly translates as “a man who deludes himself into being much bigger than he really is”, suggesting his ego.
- Since his arch-nemesis Zenitsu was only able to use the First Style of Thunder Breathing, other styles of Thunder Breathing were not revealed until Kaigaku used them. But other styles used by Kaigaku likely won’t be his original styles, as Kaigaku is now a demon with the ability to use a Blood Demon Art. Therefore, other styles used by Kaigaku were likely corrupted.
- Kaigaku and Kokushibo are the only known demon slayers who became demons. Coincidentally, Kokushibo made Kaigaku a Demon with his own blood.
