Ayumu Momose is an 18-year-old university student with light brown hair styled in a long ponytail featuring an ahoge. She stands 154 cm tall, weighs 45 kg, and has body measurements of 85-58-86 cm, green eyes, medium breasts, and a young-adult physique. Her attire frequently includes uniforms, miniskirts, thigh-high stockings, waitstaff clothing, yukatas, and casual wear like t-shirts or pajamas. As the childhood friend and girlfriend of Kensuke Udai, she met him in high school where he was her senpai. She confessed her feelings shortly before his graduation, initiating their romantic relationship. After her high school graduation, they shared an apartment until Kensuke relocated for a six-month work assignment, limiting their interactions to weekend dates. His absence triggers profound loneliness and emotional vulnerability in Ayumu. Her deredere personality makes her openly affectionate yet shy. This fragility, compounded by Kensuke's absence, leads to an affair with her university senpai, Seijun Sannomiya, following a drunken encounter. Seijun exploits her loneliness and dissatisfaction, pressuring her into continued intimacy despite initial resistance. Ayumu endures intense guilt over the infidelity due to her enduring love for Kensuke, though her resolve falters as the affair persists. The central conflict revolves around whether Kensuke discovers her unfaithfulness.