Tōko Hizuki serves as the school nurse and is Ayana Hizuki's older sister. She possesses violet hair, aubergine eyes, and notably large breasts akin to her sister's. Her ability to become invisible via bandages facilitates mischievous acts. Tōko exhibits a naughty, carefree, and provocative personality, often wearing revealing clothing to draw student attention—particularly targeting male protagonist Haruki Komaba. She teases Haruki, frequently leveraging her breasts during these interactions.

Within her familial dynamic, Tōko engages in playful or sexually charged physical contact with Ayana, including groping her sister's breasts or administering spankings. As a chemist, she develops experimental potions such as obedience serums or sensitivity enhancers.

Her background reveals she was entrusted by her sister—Ayana's mother and a genetic researcher—to care for Ayana after her creation through human-canine genetic splicing. This duty involved providing Ayana a semblance of normal life.

Tōko actively intervenes in the rivalry between Ayana's Chemical Science Club and Airi Kuze's Mechanical Science Club. Using her scientific expertise, she crafts situations to advance Ayana's interests while complicating Haruki's club decision. Her experiments frequently catalyze chaotic and sexually charged scenarios among central characters.

Her role remains consistent as a supporting character across the original OVA and source manga, instigating comedic and erotic situations through provocative behavior and scientific interventions. No significant character development or backstory expansion occurs beyond these appearances.