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 dislike him, he asked his close friend whether he has any idea, but he only gets a reply in an incomprehensible way, “you still haven't noticed it, huh”.

On a certain day, Reiji loses in a game and has to listen to 3 of Sakuya’s wishes. Reiji goes along with it even though he has doubts, but could he gradually begin to understand Sakuya’s true feelings…?

This is the story of super dense youth and a can’t-be-honest older sister until they become a lovey-dovey married couple.