Yase is a demon and a governing member of the Council of Three in the Mirror Capital. An upper-class demon of refined demeanor, she frequently hosts tea parties at her estate in the forbidden zone outside the city. Her appearance is marked by pale skin, yellow curly hair, and black eyes that possess a sparkly, demon-like quality. She customarily wears a large pink dress with a red ribbon on the back, flowy sleeves, and a matching pink hat also adorned with a red ribbon.

She was created by the priest Myōe, who used his power to bring drawings to life. Myōe specifically drew her to be a sibling for the war orphan Yakushimaru, whom he and Lady Koto had adopted. Yase, alongside her brothers Kurama and Yakushimaru, became part of a family that lived happily in the Mirror Capital until their parents departed. In the wake of this absence, the three siblings formed the Council of Three to oversee the city as they awaited their parents' return.

Yase holds a deep affection for her mother, Lady Koto, and longs for her return. She generally presents herself as polite and refined, yet she possesses a volatile temper. When sufficiently provoked, this anger can trigger a transformation into a raging monster, revealing the stark contrast between her civilized demeanor and her inherent demonic nature.

The environment of the Mirror Capital shifts according to the emotions of its inhabitants, and Yase's own emotional state contributes to these changes. Her personal journey is woven into the overarching narrative of family separation and reunion. She undergoes significant emotional development upon her family's eventual reunion, which also helps resolve underlying tensions within the group.

Her abilities are consistent with her demonic nature. Her role emphasizes themes of familial bonds, waiting, and the dichotomy between her aristocratic presentation and her inner ferocity.