Futakaku Kankei.

Twin sisters Airi and Himari have always shared an exceptionally close bond, to the point of being virtually indistinguishable from one another. Throughout their childhood, they mirrored each other in every detail, wearing identical hairstyles, matching outfits, and the exact same accessories. Their inseparable connection defined their lives, creating a sense of unity that seemed unbreakable as they grew up together. However, the transition into high school marks a sudden and unexpected shift in their relationship. As they navigate this new chapter of their lives, Himari begins to quietly distance herself from her twin. The once-unshakable closeness they shared starts to fracture, leaving Airi confused and increasingly isolated by her sister's emotional withdrawal. Unable to ignore the growing rift between them, Airi finally confronts Himari, pressing her for an explanation regarding her sudden change in behavior. When cornered by her persistent sister, Himari reveals the underlying reason for her departure. She confesses that she has made a solemn promise to an upperclassman—a commitment she is determined to fulfill no matter the cost. This revelation introduces a profound conflict between the twins, testing the limits of their lifelong devotion and forcing both sisters to grapple with the changing nature of their bond.