Mikami Yuka

A third-year student at Seifu Academy. She’s in the same class as Marina. She is not a virgin.
In exchange for medical treatment for her parents, she serves as a test subject for the protagonist’s research; she shows no signs of reluctance and calmly accepts his requests.
Because she places absolute trust in the protagonist, she is inwardly unsettled by the arrival of Marina and Airi.
The protagonist took her virginity, and she has no experience with sexual intercourse with anyone other than him.
By the way, she has small breasts.