Kagaya Ubuyashiki/Synopsis

History
Kagaya was born into the Ubuyashiki clan. As a male born into the family, he was cursed to suffer an illness that slowly afflicted his entire body, due to Muzan Kibutsuji's blood relation with the clan. He became the leader of the Demon Slayer Corps when he was only 4 years old.

He first met his wife Amane when he was 13 years old and she was 17. She was specifically selected by a priest to become Kagaya's wife in order to care for his medical needs, but he only agreed to the marriage on the condition that she cared for him on her own free will.

When Kagaya was fourteen, he learned of a monk who was imprisoned for the deaths of six children under his care. Having realized that the monk was innocent and that the deaths were the result of a demon's attack, he intervened and prevented his execution, after which he became a Demon Slayer. This man, Gyomei Himejima, would go on to become one of Kagaya's most trusted confidants and the Stone Hashira of the Demon Slayer Corps. Sometime later, Kagaya met with a shinobi turned Demon Slayer named Tengen Uzui. Tengen had left his past behind, unwilling to become a ruthless killer like his father and brother. Kagaya praised Tengen for his decision, citing the great difficulty the young man must have endured to leave everything behind and accept a new direction in life. Touched by his words, Tengen expressed gratitude and become intensely loyal to Kagaya, later becoming the Sound Hashira.

During Sanemi Shinazugawa's first meeting as a member of the Hashira, he harshly insulted Kagaya for sending Demon Slayers under his command to die with an unconcerned smile on his face, angering Gyomei and Tengen, and worrying the Flower Hashira Kanae Kocho. Despite their concerns, Kagaya allowed the Wind Hashira to continue speaking his mind. When Sanemi accused him of being a weak leader to the Demon Slayers, Kagaya apologized to him, shocking the angered young man. He explains that due to his condition, he cannot even swing a blade ten times without his blood pressure becoming dangerous, and if he wanted to become strong, he would but cannot. He apologizes to Sanemi for making him do what is difficult, the once furious Hashira calmed by his voice and gentle expression. Kagaya would go on to tell Sanemi that if he believed the Demon Slayers were pawns to him, that he too was once since his replacement is already set. Additionally, he explains that he is not above anyone; the Demon Slayers simple treat him with respect but he does not have to. He apologizes for summoning him soon after the death of his partner Masachika Kumeno. In disbelief in hearing his name, Sanemi would be told by Kanae that Kagaya remembers the names of every single Demon Slayer under his command. The Corps leader gives him Masachika's will to read, causing him to tear up. Since seeing his genuine dedication to the Demon Slayer Corps, Sanemi has also become extremely loyal to Kagaya.

During Muichiro Tokito's career as a Demon Slayer, he was visited by Kagaya and Amane during his recovery. Seeing the confused and heavily wounded boy, Kagaya extended words of advice to him, telling Muichiro that one's survival is foremost and that once he survives, lost memories will come back.