Miyuki Nanase, operating under the alias Yukiko Hoshino, serves as a significant antagonist. Her origin stems from an affair between her mother and her legal paternal grandfather, leading to severe abuse from her de jure father—who was biologically her half-uncle. This abusive environment forged her traumatic upbringing and psychological instability.

Guided by Yakumo Saitou's biological father, she murdered her abusive family to escape her suffering. This act cemented her fanatical loyalty to him, whom she regards as her savior. She harbors intense jealousy towards Yakumo fueled by his father's attention, driving her antagonism.

Physically, Miyuki possesses blonde hair styled in a high ponytail, dark golden-yellow eyes, and distinctive pointed oval-shaped glasses. Her consistent appearance across adaptations reinforces her role as a calculated and recognizable adversary.

Her personality combines physical strength, mental instability, and unwavering devotion to Yakumo's father. She exhibits sadistic tendencies, particularly targeting Yakumo, whom she professes to "love" while desiring to "break" him. This duality manifests in her pleasure from inflicting pain alongside a detached, professional approach to violence.

Throughout the series, she acts as the chief accomplice to Yakumo's father, aiding his criminal endeavors with methodical precision. Her motivations arise from a desperate yearning to achieve her goals and find happiness with him, though her actions escalate into extreme violence. Her arc culminates in numerous brutal acts, including murder and psychological manipulation, establishing her as a persistent threat.

Her name holds thematic significance: "Miyuki" (美雪) combines "beautiful" (美) and "snow" (雪), while "Nanase" (七瀬) means "seven" (七) and "swift current" (瀬). The alias "Yukiko Hoshino" (星野由紀子) incorporates "star" (星), "field" (野), "reason" (由), "chronicle" (紀), and "child" (子), reflecting layered identities.