Seto Koharu, the younger sister of Shuuhei Seto, lives with her brother, their parents, and Seara—a sixteen-year-old who joins their household. Initially resistant to Seara’s presence and uneasy about her closeness to Shuuhei, Koharu’s hostility recedes when Seara aids her in pursuing a boy she admires, sparking a tentative friendship.

As classmates and roommates, their dynamic shifts from friction to camaraderie after Seara’s pivotal role in Koharu’s romantic endeavors. Koharu’s protectiveness toward her brother initially fuels her wariness, but she gradually softens upon recognizing Seara’s earnest attempts to integrate into the family.

Koharu’s arc centers on her transition from skepticism to trust. Early interactions brim with tension, while later moments highlight her collaboration with Seara to navigate social and romantic hurdles. The narrative focuses solely on her school life and domestic relationships, particularly with Seara and Shuuhei, without expanding on her history or future beyond these settings. Her story remains confined to the original OVA, anchored in familial and peer connections.