Sei Tōdō, a composed and gentle tutor of grammar, literature, and social sciences, instructs alongside his extroverted twin brother Kai, who teaches math and science. Their surname, “wisteria temple,” anchors them to tradition and order, while Sei’s given name—merging “oath” and “only”—signals his steadfast loyalty and singular purpose. Years before the main events, he survived the same harrowing ritual trials as the protagonist, an experience that etched lasting anguish into his psyche. When he defies the ritual’s strictures to shield the protagonist, he is petrified as punishment, revealing both his protective drive and the system’s unforgiving nature. This act deepens his bond with Kai, evolving into a romance marked by a bittersweet farewell kiss in the OVA after Kai relinquishes his humanity to replenish Sei’s life force.

Later, Sei departs for a German university via an exchange program, maintaining an emotional tether to his past despite physical distance. His return coincides with the transient reappearance of Kai’s mirror shard, permitting one final reunion before the fragments reunite with the mirror—a reflection of Sei’s enduring role as a conduit between the human and supernatural worlds.

His arc grapples with the tension between obligation and personal yearning, particularly amid the ritual’s moral ambiguities. The dissonance between his tranquil exterior and inner turmoil amplifies the narrative’s focus on sacrifice and the perpetuation of inherited trauma.