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.

When he was only 4 years old, his father and the previous family head, was a sensitive man who committed suicide at the age of 19 because he could not bear the thought of his troops getting hurt or dying. The children he had were all boys, and he lost all of them except Kagaya who assumed the role of Oyakata-sama after his father's death.

When he was 13 years old, he first met his wife Amane. 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 14, he learned of a blind monk who was imprisoned for the deaths of eight 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. This man, Gyomei Himejima, would later become a Demon Slayer and go on to become one of Kagaya's most trusted confidants and the Stone Hashira.

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 became intensely loyal to Kagaya, later becoming the Sound Hashira.

During his first meeting as a member of the Hashira, Sanemi Shinazugawa 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.

Kagaya encountered a young Demon Slayer who was ostracized for her abnormal strength and unusual traits. Taking pity on her, the Demon Slayer leader told her that her strength was a beloved gift from the gods and anyone who spoke ill of her was merely envious. The girl, Mitsuri Kanroji, was deeply touched by Kagaya's words, crying and bowing before him in relief and thankfulness. She would later go on to become the Love Hashira of the Demon Slayer Corps. 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. At a later time, he then asked him not to be so rigid with his perception of the world, telling the boy if he can find his true self, he can become unstoppable.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 the Water Hashira Giyu Tomioka and Insect Hashira 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. Later that night, Kagaya and the Hashira continued their meeting. All were angered by the growing threat of demons on humanity and noted the declining skill in the newer Demon Slayers. Gyomei addressed the issue that since only people who know about demons become Demon Slayers, they cannot bolster their numbers easily. When Kyojuro noted that Tanjiro has encountered a Twelve Kizuki early into his career, Kagaya mentions that Lower Five's drastic actions on the mountain would mean that Muzan was no where near, due to always creating an distraction. While disappointed in demons becoming more dangerous than ever before, Kagaya expressed his faith in the Hashira, deeming them the strongest since feudal times and looking forward to his "childrens'" success. Later that night, while alone in his quarters, Kagaya vowed that they will kill Muzan without fail.

Mugen Train Arc
When he learns that not one of the 200 passengers aboard the Mugen Train was killed in the demon's attack, Kagaya praises the fallen Kyojuro for his sacrifice. He is not saddened by the Hashira's death, as he remarks that he himself does not have long to live for and will be joining him and the other deceased soon.

Entertainment District Arc
Following Tengen's mission in Yoshiwara that resulted in the deaths of the Upper Rank demons Daki and Gyutaro, Kagaya, whose health has visibly deteriorated and worsened, is informed of the news by a Kasugai Crow in his estate. He is ecstatic to learn the accomplishment was performed by Tengen, Tanjiro, Nezuko, Zenitsu Agatsuma, and Inosuke Hashibira. Amane assists Kagaya; despite his severe coughing now causing him to bleed, he states that an Upper Rank's death has not occurred in over a hundred years and grabs his wife's hand in joyous realization, telling her that the death is an omen for the coming future changing in their favour.

He remarks that the consequences of such a death will spread far and in time, they can kill Muzan once and for all. His coughing suddenly begins to worsen and Amane orders their children to fetch his medicine, as they worriedly call out to their dying father.

Swordsmith Village Arc
After saving a young swordsmith from a fish-like demon, Muichiro initially moves on ahead to protect the Swordsmith Village, having realizes that the demon was actually a Blood Demon Art. The smith, Kotetsu, instead begs him to save two swordsmiths who are alone and fending for themselves from the demons. Though at first he ignores him to go his way, Muichiro suddenly remembers a moment when Kagaya and Amane had visited him following an attack by a demon that left him severely wounded. Kagaya, noticing the boy's confusion, calmly tells him that his survival is paramount. Should he survive, his lost memories will come back too, telling him to not miss the opportunity, as even trivial events can trigger them to come back. Taking his leader's advice, Muichiro forgoes his initial plans and grabs Kotetsu to rescue Kozo and Hotaru. Though he is initially hesitant to abandon his post, Muichiro trusts in Kagaya's advice and remarks he can succeed as the Mist Hashira recognized by him.

During his confrontation with Muichiro, Gyokko, after trapping the Mist Hashira in his Water Prison Pot, told him that one of their objectives is to kill Kagaya, along with destroying the Swordsmith Village, in the hopes of crippling the Corps of leadership and weapons.

At a later time, after killing Gyokko, Muichiro recalls a past meeting he had with Kagaya, who gentle asked him to ease his rigid perception of his surroundings. He tells the Mist Hashira that once he finds his true self, he can become unstoppable. True to Kagaya's advice, Muichiro realizes that by focusing his boiling rage on a target without losing himself, he was able to defeat Upper Rank Five. While stunned from Zohakuten's Compressed Sound Waves, Mitsuri reminisces on her past and how she gradually realized that acceptance from others of her unique traits would be nearly impossible. Upon joining the Corps however, Kagaya immediately told her that she was special, and that anyone who speaks ill of her was simply afraid of her strength. Hearing whole-hearted accepted of her true self brings the once uncertain and unconfident young woman to tears, bowing and thanking Kagaya for his words.