Yū Yagami presents as a warm, caring nurse at a hospital, her brown hair styled in chest-length twintails with ribbons and her blue or green eyes observing patients from beneath her standard nursing cap.

This professional facade masks severe childhood trauma: prolonged sexual abuse by her home tutor created an emotional dependence. His subsequent suicide left her detached and incapable of genuine connection, establishing patterns of psychological disturbance and compulsive sexual behavior.

Exploiting her role, she manipulates patients, particularly the injured novelist Jirou Yotsuya. Under hospital administrator Reika Mikage's direction, she participates in experimental treatments serving as pretexts for non-consensual acts, employing methods like sleep deprivation and extreme sexual manipulation described as "sucking the soul dry" from victims.

Her behavior escalates to include participation in violent acts, including murder, while maintaining outward normalcy. She exhibits no remorse, walking "over corpses" to fulfill her compulsions, a direct correlation between her past trauma and the merging of apparent professionalism with predatory actions.

Her depicted storyline offers no redemption or substantive evolution, concluding with confinement in a psychiatric institution following multiple fatalities, cementing her portrayal as irredeemably damaged by her abuse history.