Imouto no Shinyuu? Mou Ore no Onna Tomodachi? Nara, Sono Tsugi wa—?

Touma Itohara is not interested in love. He is comfortable with relationships where lines are drawn, and there's no doubt that it's fulfilling for him.

"Oniisan, if you haven't left yet, let's go home together."

But recently, there was a little change that occurred in his daily life: Rin Mamiya. She is his sister's best friend. A girl who often comes to their house... At first, their relationship was just that. But now that they go to school together, they go home together, and hang out together on holidays as a matter of course. They also also started to hang out on weekends, and their relationship grew stronger and stronger. Even so, Touma only treats her as a "friend of the opposite sex." But on the other hand, Rin seems to have her own feelings about this relationship...?