As a human Kaigaku was a fit young man who had black hair and turquoise eyes. As a demon he gained black, flame-like markings on his face, fangs, and pale skin. As an upper moon, he gained kanji on his eyes.
He usually wears a black kimono over his demon hunter uniform, that has yellow markings on it.
Kaigaku is a very prideful, to the point that he’ll serve anyone who recognizes his strength. He’s very intelligent learning breathing techniques almost flawlessly. His ingenuity was only strengthened by his demonic transformation, as he used the special powers of his blood to improve his breath style. He hates weakness, especially when it obstructs something he wants, as seen with Zenitsu.
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. He taken in by Jigoro Kuwajima, along with Zenitsu, and taught the Breath of Thunder style. He became determined to become Jigoro Kuwajima’s successor and the new Thunder Pillar. Jigoro Kuwajima however said that Zenitsu would become his successor. This, along with his weak power and timid nature, caused him to hate Zenitsu.
Later he went on a solo mission to hunt down Kokushibo, though he was to weak to even scratch him. After the defeat he kneeled before Kokushibo and begged for his life. Kokushibo offered Muzan's blood to turn Kaigaku into a demon, which he accepted. As a demon Kaigaku ate a multitude of people and came abnormally powerful. And after the deaths of Daki and Gyutaro, he became Upper Moon Six of the Twelve Demon Moons.
However, when he finally had a rematch against Zenitsu, 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 admitted that Flaming Thunder God was his own making, in order to fight alongside Kaigaku as equals, had him not turning into a Demon. Kaigaku notes that Zenitsu was going to die from severe exhaustion only to be saved by Yushiro in the last seconds. Ultimately, Kaigaku head screams in anger as he disintegrated.
Abilities and Powers
Enhanced Breath Techniques: As a demon, Kaigaku's Breath of Thunder techniques have been 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 given another year, Kaigaku's strength would have crushed Zenitsu on the spot.
Breath of Thunder Style (
- Second Style: Lightning Ball (
弐ノ 型 稲 魂Ni no kata: Inadama?) - A series of five straight attacks all performed in a single moment.
- Third Style: Buzzing Mosquito Thunder (
参ノ 型 聚 蚊 成 雷San no kata: Shiubun seirai?) - A spinning wave attack in all directions.
- Fourth Style: Distant Thunder (
肆ノ 型 遠 雷Shi no kata: Enrai?) - A ranged strike at a distance.
- Fifth Style: Heat Lightning (
伍ノ 型 熱 界 雷Go no kata: Retsu kairai?) - A slashing attack that cracks skin and burns flesh.
- Sixth Style: Electric Lightning Shower (
陸ノ 型 電 轟 雷 轟Roku no kata: Dengou raigou?) - A series of strikes that crack the target's body.