Kaigaku is a supporting antagonist in the Infinity Castle Arc of Demon Slayer: Kimetsu no Yaiba. He is a demon affiliated with the Twelve Kizuki, holding the position of Upper Rank Six, replacing Daki and Gyutaro after their deaths. Kaigaku was a former Demon Slayer and Zenitsu Agatsuma's senior disciple under the former Thunder Hashira, Jigoro Kuwajima. However, sometime during Hashira Training, Kaigaku went on a mission and encountered Upper Rank One, Kokushibo, quickly surrendering to him to save his own skin and later becoming a member of the Twelve Kizuki. As a human, Kaigaku is a muscular young man of average height who has short, spiky black hair, with two thick strands framing his face, and a pair of dull, cold turquoise eyes framed by thick, arched eyebrows. He has an unfriendly face, that is always set in a semi-permanent snarl. After becoming a demon, he gains many demonic traits like four black flame-like markings on both sides of his cheeks, elongated canines that resemble fangs, sickly pale skin, sharp blackened nails, and pointed ears. By becoming a member of the Twelve Kizuki and rising to the position of Upper Rank Six, his eyes are etched with the kanji of "Upper Rank" and "Six". When he was still human, who is still undergoing training under Kuwajima, he is shown to be wearing a black kimono with a white collar, worn loosely to expose his chest and his signature magatama-necklaces around his neck. When he became a Demon Slayer, he wore the Corps' standard uniform consisting of a black gakuran-like jacket, a pair of equally black tattsuke-hakama pants held up by a white belt, tucked into white kyahan wrapped around his ankles, and a pair of tabi socks and zori. When he became a demon, he is shown to be wearing his signature kimono secured with a blue obi over his Demon Slayer uniform that has yellow flame-like markings near the chest area. His forearms are seemingly wrapped in kyahan-like material. He also wears multiple necklaces on his neck and wrist with green colored strings that hold together blue magatama-like pendants that were yellow in color when he was human. Though outwardly brash and arrogant, Kaigaku is in fact cowardly and selfish. Like many orphans living on the streets, he grew up resorting to thievery and other immoral means to survive. He even goes so far as to sacrifice the lives of all the orphans under Gyomei's care to save his own skin. This has caused him to develop an intense form of self-preservation, often doing what in his own best interest at the expense of others. At some point after this, Kaigaku met and became the disciple of Jigoro Kuwajima. Because of his great talent with a sword, he was poised to become the next Thunder Hashira. However, unlike other orphans who join the Corps, Kaigaku never developed a desire to slay demons or protect people. When confronted with a stronger opponent, Kaigaku quickly surrendered. Furthermore, he seems to have developed a sense of hypocritical self-righteousness. He called Zenitsu Agatsuma worthless for having no dignity or perseverance and was repulsed at the idea of succeeding Jigoro along with him. Kaigaku also seems to have a twisted perspective of 'right' and 'wrong', believing that those who judge him right (i.e., say he is worthy) are good and those who judge him wrong and have a low estimation of him are evil. Additionally, he also has an inflated ego, viewing himself as a special and gifted person. In the end, after being beheaded and being at death's door, Kaigaku remains unrepentant and is pleased to know that at least Zenitsu would die with him. However, Yushiro arrives and begins mocking him that he will be given nothing since he never learned how to give, and Kaigaku's fate was sealed once he decided to let his desire inflate and take control of him. Upon seeing Yushiro rescue Zenitsu, Kaigaku screams in anger as the remainder of his body disintegrates.