Sora Aoi has always been close with his siblings. But while Nami may be his twin sister, he and his older sister Aki share a different kind of relationship. What began as an encounter with Aki in the bath led to a startling realization for Sora: he is in love with her. But while Sora is hesitant about his forbidden feelings, Aki confesses that she has always loved him. With their affections out in the open, the two begin a secret relationship.

Aki-Sora was bundled with the release of the limited-edition third volume of the manga.