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 one as well 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. Muichiro would later become Kagaya's Mist Hashira.

Final Selection Arc
Following Final Selection, Kagaya is informed that there were a total of five survivors. Satisfied that the number of his "children" has increased, he wonders what kind of Demon Slayers they would eventually become.

Mount Natagumo Arc
During the mission to Mount Natagumo, a Kasugai crow flies from the mountain to the Demon Slayer Corps headquarters to inform Kagaya of the massacre that has occured. Believing the Twelve Kizuki are involved and the deployment of the Hashira to be necessary, he sends Giyu Tomioka and Shinobu Kocho to intervene.

Rehabilitation Training Arc
During Tanjiro Kamado's trial by the Hashira for harbouring his demon sister Nezuko, Kagaya appears for their semi-annual meeting and comments on the lovely weather. The Hashira bow before him and Sanemi extends greetings to Kagaya. The Wind Hashira then questions him on the circumstances regarding Tanjiro but to the Hashira's surprise, Kagaya has accepted Tanjiro's situation and wishes for them to accept it as well. Gyomei, Tengen, Sanemi, Flame Hashira Kyojuro Rengoku, and Serpent Hashira Obanai Iguro disagree due to the nature of demons but Muichiro and Love Hashira Mitsuri Kanroji accept it, while Shinobu and Giyu stay silent. With his answers, Kagaya informs them about a letter from Sakonji Urokodaki, the former Water Hashira, writing that he believes Nezuko to be fully able to resist attacking humans and that he, Giyu, and Tanjiro would commit seppuku should it ever happen.

Kyojuro and Sanemi still disagree and claim that it cannot be proven Nezuko won't attack but Kagaya points out that it is also unsure if she would, and citing the fact that 3 people are putting their lives on the line, he asks them to provide a stronger argument. Additionally, he adds that Tanjiro has met Muzan Kibutsuji in the past and has had demons sent to kill him. Though the Hashira realize how important he is, Sanemi attempts to prove Nezuko's demonic nature by stabbing her and taunting the demon with his own blood. He apologizes to Kagaya for his rude intrusion and enters the headquarters to open her box. When Nezuko proves herself capable of resisting his blood, Kagaya, being blind, questions what has happened, to which he is informed that Nezuko turned away from Sanemi.

Believing this to be indisputable proof, Kagaya is pleased and addresses Tanjiro, who notes the floating feeling he experiences hearing his voice. Kagaya informs him that there will be those who would not accept her sister and tells him to prove his worth by killing a Twelve Kizuki. Tanjiro, feeling elated by his acceptance, vows to kill Muzan, though the Master humorously implores him to start small with a Kizuki demon. He then addresses Sanemi and Obanai, gently asking them not to torment those younger than themselves, which they acknowledge. With Tanjiro's trial now finishes, Shinobu takes custody of him and Nezuko to live in her estate. Kakushi bow before Kagaya and the Hashira and quickly take the siblings away. However, before Kagaya can continue speaking, he is interrupted when Tanjiro runs back and tries to headbutt Sanemi for stabbing Nezuko, asking for Kagaya's permission. He is quickly knocked down by Muichiro flinging pebbles and taken away again, but before the boy leaves his presence, Kagaya asks for Tanjiro to "give Miss Tamayo my regards". surprising him.