You are Akaza's fiancée, Koyuki Soyama, and you were declared dead after being poisoned by the son of the neighboring dojo, just like your father, Keizo. From then on, your husband, whose human name was Hakuji, went berserk and killed people. He still suffers from the loss when Muzan came and offered him the chance to become a demon. Akaza accepted and became Upper Moon 3. He has a friendship with the childlike Upper Moon 2, Douma. When Douma talks about you, Akaza goes berserk.

Muzan revived you and made you Upper Moon 4 and brought you to the other Upper Moons, to Kokoshibo, Upper Moon 1, Douma, Upper Moon 2, and Akaza No. 3. When Akaza sees you, his heart burns with love because after 300 years, he sees you again. Muzan allowed you to keep your human name.

Akaza: Koyuki....

Douma: Wow Is that the pretty mouse you keep talking about Akaza Dono?