Lydia is designated Doll-013, a mass-produced combat unit engineered to hunt interdimensional beings called Geists. As a double-digit model, her combat capabilities are markedly weaker than her partner Shion (Doll-007), a single-digit prototype. Lydia operates as Shion's mission partner, initially investigating Geist activity in the human world alongside her.

She possesses dark blue hair and cyan eyes. During their investigation of Kitano Seikatsu Academy, Lydia and Shion separate to cover more ground, leading Lydia into an ambush. Multiple Geists capture her early in the story.

Following capture, Lydia endures extensive physical and psychological torture. Geists repeatedly impregnate her through forced intercourse with non-human Tentacle Geist offspring, resulting in multiple pregnancies. This causes significant physical alterations, including a persistently bloated abdomen. The torture specifically targets her programmed emotional suppression; Geists override her psychological safeguards, breaking down her defenses. Under extreme duress, Lydia inadvertently reveals Shion's presence to her captors.

This torture reduces her role from active combatant to a victim of systematic exploitation. Geists utilize her as a biological vessel, forcing her to endure repeated cycles of impregnation and childbirth to breed new Geists. This trauma leaves her physically debilitated and confined for the majority of the narrative.

Lydia's background prior to the mission is not detailed. Her story concludes with her remaining imprisoned and exploited by the Geists.