Riku Seya, typically called Seya, enters as Maho Nitori's junior high classmate. He initially mistakes Maho's cross-dressing younger sibling, Shuichi, for a girl upon meeting them at the Nitori home, developing an attraction based on this error. Aware of Seya's misconception yet harboring her own feelings for him, Maho arranges a meeting. This leads Seya to briefly date Shuichi while believing them to be a cisgender girl. Discovering Shuichi's transgender identity, Seya angrily confronts Maho, feeling deceived. He later reconciles with both Maho and Shuichi.

After this reconciliation, Seya and Maho begin a romantic relationship, becoming an official couple following Maho's confession. Their bond deepens, shown through public affection like holding hands walking home. Seya exhibits loyalty and protectiveness towards Maho, especially when she faces social pressure and rumors after Shuichi attends school in a girl's uniform. He actively supports her, offering to be her "bodyguard" and working to counter prejudice.

To foster understanding and reduce the stigma affecting Maho and Shuichi, Seya organizes a gender non-conforming event for the school festival. He conceives and manages a "Maid Café" where male students wear traditionally feminine attire like blouses and skirts, while female students dress in formal, traditionally masculine attire as waiters or butlers. This gender-reversed event succeeds, boosting Seya's reputation as an organizer. Portrayed as generally even-tempered and polite, Seya retains some initial awkwardness with Shuichi due to their past. Over time, he shows increasing care and empathy towards both siblings, motivated by a sense of fairness and duty to support his friends against societal injustice.