Yuu Yagami is the primary nurse assigned to oversee the care and experimental treatment of Jirou Yotsuya, a writer hospitalized following a severe accident. She operates under hospital director Reika Mikage, who designates Yotsuya as a test subject for unverified drug treatments due to his lack of family connections and perceived physical suitability. Yuu's duties involve administering experimental medications, monitoring their effects on Yotsuya, and providing sexual acts as part of a Mikage-authorized reward system for his compliance.

Her character presents a paradoxical duality: she projects the image of a warm-hearted, dedicated medical professional while simultaneously exhibiting compulsive nymphomania. This compulsion manifests as relentless sexual demands on her assigned patient, extending beyond clinical directives into personal obsession. She subjects Yotsuya to continuous sexual encounters without regard for his physical or mental deterioration, preventing adequate rest and accelerating his decline. Her behavior escalates to increasingly extreme acts throughout the experimental process, consistent with the director's permission to exploit hospital staff for Yotsuya's gratification.

Visually, Yuu possesses brown hair styled in twintails secured by ribbons and blue or green eyes. She typically wears a standard nurse uniform, including a cap, presenting an appearance of conventional medical professionalism that contrasts with her sexualized conduct.

Within the narrative, her function centers on facilitating the drug trials and enabling Yotsuya's descent into physical and psychological torment through non-consensual sexual exhaustion. The experiments and her actions form a continuous cycle of exploitation. The narrative scope focuses solely on her role within the hospital's protocols, without exploration of her personal history, motivations, origins, internal conflicts, or experiences outside the depicted events.