“I always side with those who properly acknowledge me whenever and at any time.”
- Japanese Voice Actor: Yoshimasa Hosoya
- English Voice Actor: Alejandro Saab
- Age: N/A
- Manga Debut: Chapter 34 (Human) Chapter 143 (Demon)
- Anime Debut: Season 1 Episode 17
- Race: Human / Demon (Now)
- Gender: Male
- Status: Deceased
Kaigaku has a series of treacherous acts that harm others to the point that it leads to their death. The first one was when he was an orphan under Gyomei Himejima. Before Gyomei became a Hashira or a member of the Demon Slayer Corps, he was taking care of orphan children.
Kaigaku is one of those orphans he took care of. Kaigaku would always steal the temple money. One time, he was banished from the temple. While he was walking in the forest, he met a demon. He told the demon about the temple and how to enter it to save his life.
Kaigaku even led the demon to enter the temple, leading to the children getting killed and Gyomei becoming imprisoned.
After the event at the temple, Kaigaku wandered around. He becomes strong so that he can survive. He would do any means necessary to feed himself. He does not care if he has to bow down or kneel for him to survive.
Jigoro took in Kaigaku and Zenitsu under his wings and trained them to be demon slayers. Kaigaku learned the Thunder Breathing style from a former Hashira, Jigoro. Kaigaku had always wanted to be the successor of Kaigaku and become a Thunder Hashira.
Jigoro told Kaigaku that he wanted Zenitsu to be his successor instead. This angered Kaigaku, and he felt like Jigoro is favoring Zenitsu, who is a timid boy.
Kaigaku never learned the first form of the Thunder Breathing style. He mastered all the forms from 2nd to the sixth form. When Zenitsu was in Nagatumo mountain, he remembered how Kaigaku would treat him for calling their sensei “gramps.”
Kaigaku believed that Jigoro highly favored Zenitsu because Zenitsu learned the first form of Thunder Breathing. When Kaigaku became a member of the Demon Slayer Corps, he sought out Kokushibo. When he met him, he was astounded by his power and kneeled before him to make him a demon.
When Zenitsu was doing the Hashira training, he received a letter saying that Kaigaku became and demon and Jigoro committed seppuku to atone. When Kaigaku became a demon, he started eating many people.
He also received a huge amount of blood from Muzan since those who practice breathing techniques have more resistance to the blood of Muzan. With how he preyed on humans and the blood he received from Muzan, he became strong enough to be a member of the Upper Moon.
He replaced Daki and Gyutaro in the Upper Ranks when they were killed in the Red Light District.
During the Infinity Castle Arc, Kaigaku faced Zenitsu head-on. Zenitsu told Kaigaku about the death of their teacher, Jigoro, saying he committed seppuku because Kaigaku became a demon. Kaigaku seems unfazed by it.
He even accused Jigoro of favoring Zenitsu, and Jigoro was not a good person, for he did not praise him. Zenitsu started to get angry with Kaigaku and told him he was a piece of trash that could not even master the First Form of Thunder Breathing style.
Kaigaku was beheaded by Zenitsu using the Honoikazuchi no Kami (Flaming Thunder God), the seventh form of the Thunder Breathing Style. Kaigaku continued accusing Zenitsu of being favored by their teacher for learning another style, but Zenitsu told him that he created this style to fight him as equal.
As Kaigaku disintegrated, his head screamed in anger and frustration at Zenitsu. Kaigaku was the second demon slayer to become a demon. He is strong enough that it is noted that if Kaigaku and Zenitsu had fought a year after the Infinity Castle Arc, Kaigaku would have killed Zenitsu in a blink of an eye.
When Kaigaku was still human, he was a muscular man with an average stature. He has black hair and a pale complexion. His eyes were turquoise in color. He still had the same structure when he became a demon but gained some demonic traits.
He got fangs, sharp black nails, and pointed ears. His cheeks also have flame-like markings on both sides. His once turquoise eyes now have the kanji for Jogen (Upper Rank) and Roku (Six) since he became a member of the Upper Moon and his rank was 6.
When he was human, he was wearing a yellow magatama. These pendants become blue when he becomes a demon. His clothes did not change since he was human until he became a demon. He still wears a black kimono with a blue obi over his Demon Slayer Corps uniform, with a yellow flame-like marking over his chest.
Kaigaku is outwardly brash and arrogant. He would look down on anyone he thinks has lower capability than him. He dislikes favored people, even if the flavor is justly given. Despite his arrogant exterior, he is a coward.
He would not think twice about selling his comrades to save his life. He is a hypocrite and self-righteous. He has a twisted perception of who should be respected and what is right and wrong. He thought of himself as gifted and special, resulting in an inflated ego.
Being a member of the Demon Slayer Corps, Kaigaku has excellent swordsmanship. He is extremely strong, having received much blood from Muzan and consumed many humans. Being the youngest of all the Upper Ranks and having just acquired his power, Kaigakku may have strength, but he does not have total control of his power and still lacks mastery of it.
His Thunder Breathing style contributed to his speed and reflexes, which were then enhanced when he became a demon.
His Blood Demon Art compliments his breathing style as he can now generate black electricity from his blood and flesh. This lightning-like Blood Demon Art can crack the flesh and burn it.
He uses his Blood Demon Art and Thunder Breathing Style simultaneously. After the fight with Zenitsu, even when Kaigaku died, Zenitsu also barely made it. Zenitsu was heavily injured and could have died if Yushiro did not save him.