Kagaya Ubuyashiki, the 97th leader of the Demon Slayer Corps and head of the Ubuyashiki family, bore a hereditary curse linked to Muzan Kibutsuji's bloodline. This curse afflicted male family members with a progressive, debilitating illness leading to physical deterioration, blindness, and death before age thirty. The disease crept visibly across his body, manifesting as shriveled, discolored skin, blackened sclera, and frequent bleeding from the eyes and mouth. By the final stages, he was bedridden, bandaged, and endured relentless agony. He assumed leadership at age four after his father took his own life, overwhelmed by the repeated deaths of Demon Slayer Corps members. Further tragedy struck his childhood when his resentful younger brother, Senri, set fire to their mansion in a suicide attempt, forcing their youngest brother Akito to participate; their mother perished trying to save them. At fifteen, Kagaya fathered quintuplets—Hinaki, Nichika, Kiriya, Kuina, and Kanata—with his wife Amane. Amane was specifically chosen to care for him, and Kagaya insisted she marry him only of her own free will. Despite his frail condition, Kagaya possessed profound leadership qualities marked by calmness, compassion, and unwavering dedication. He regarded all Corps members as his children, remembering every individual's name and visiting fallen members' graves daily until his illness prevented it. His demeanor remained serene and gentle, often smiling through pain. He demonstrated exceptional emotional intelligence and diplomacy, resolving conflicts among the Hashira and persuading even volatile members like Sanemi Shinazugawa with measured, sincere words. This stemmed from his "Soothing Voice," a trait associated with charismatic leaders. Kagaya exhibited supernatural abilities tied to his lineage. He possessed uncanny extrasensory perception, detecting the presence and strength of humans and demons despite his blindness. His family also inherited heightened instinct and foresight, enabling him to predict critical events accurately. He foresaw that decapitation alone wouldn't kill Muzan and that sunlight remained his sole weakness. He predicted the deaths of Upper Rank demons Gyutaro and Daki and anticipated Muzan's final confrontation during his last days. His most notable skill was "Emotion Masking," which concealed his intense hatred for Muzan beneath a tranquil exterior, deceiving the demon during their final encounter. Kagaya played pivotal roles in recruiting and mentoring key Hashira. At fourteen, he intervened to save Gyomei Himejima from wrongful execution, recognizing a demon's involvement in the deaths of children under Gyomei's care; Gyomei later became the Stone Hashira and a trusted confidant. He validated Tengen Uzui's decision to abandon his shinobi past, fostering Tengen's loyalty as the Sound Hashira. He comforted Mitsuri Kanroji by affirming her strength as a divine gift, leading her to become the Love Hashira. He guided Muichiro Tokito through memory loss and recruited Obanai Iguro as the Serpent Hashira despite his trauma. During Tanjiro Kamado's trial for harboring Nezuko, Kagaya overruled objections from several Hashira by accepting Sakonji Urokodaki's letter of guarantee. He defended Tanjiro's value as a witness who had encountered Muzan directly. In the Mugen Train arc, he honored Kyojuro Rengoku's sacrifice, emphasizing Rengoku died fulfilling his duty to protect all passengers. Foreseeing his own death would trigger the final battle, Kagaya orchestrated a trap by luring Muzan to the Ubuyashiki estate. He sacrificed himself and his family in an explosion to severely weaken Muzan, an act contributing decisively to the demon's defeat. Following Muzan's death, the family curse lifted, allowing his son Kiriya to succeed him and live a full lifespan.