Seiren serves Gargantua, a figure tragically linked to Jill—an incarnation of Eve within a book-world narrative. In the orphanage storyline, Seiren aids Gargantua’s obsession with Jill, whose mute, nurturing demeanor spirals into a fatal confrontation where Gargantua accidentally stabs her. Jill’s death triggers an explosive release of souma, granting immortality to Gargantua and Ritsuko, another pivotal character.

The game adaptation introduces Seiren’s mastery of sexual magic, which manipulates the protagonist’s choices in specific arcs. A cloned iteration of Seiren, engineered by her future self, emerges to traverse book worlds with two children, weaving cyclical complexity into her identity across fragmented realities.

In the anime, Seiren appears in a later arc as caretaker to Milka, a girl confined to a five-year-old’s form by a traumatic past. Residing on a secluded island with Milka and Rascalle, a white tiger, she embodies a gentler guardianship—a contrast to her earlier alliance with Gargantua, underscoring her multifaceted roles across disparate narratives.

Her documented involvement remains confined to these game and anime arcs, with no expanded appearances in other official media.