Tomodachi no Onee-san to Inkya ga Koi wo Suru to Dou Naru no ka?

A self-proclaimed unattractive gloomy high schooler, Hiramiya Reiji, gets messed around by his friend's older sister, Sakuya, every time he goes to visit him at home. Drinking his drink as she pleases, barging into the bathroom, and crawling under his blanket in the middle of the night… Thinking that perhaps he has done something to make her...