Theresia is introduced as the Count's daughter, much to Puck's shock, as the elf is given to her as a gift. She appears stiff and stoic around her father, and recoils when he attempts to touch her. Despite Puck's fear of her, she quickly assures him she won't harm him and that she knows what it's like to live in a cage, before opening up to him about her life. Describing the change her father went through seven years ago, she states that her mother was killed by a band of heretics, inspiring the demonic change in her father and his appalling bloodlust.

Soon, Puck has to leave in order to help Guts, but assures her that he will come back. However, before Puck can fulfill this promise, Theresia leaves her room on her own, inspired by Puck's own freedom, and walks in on Guts' battle with her monstrous father in Apostle form. Terrified and with her worst fears confirmed, Theresia runs away, but is taken hostage by Guts, who uses the opening presented to blast the Count's face apart and cleave off his head. Theresia cries while Guts torments her father viciously, though it is unknown if she feels he is going too far or is simply overwhelmed by the events.

Theresia is quickly reduced to hysterics as the God Hand arrive, being suggested and pushed as a sacrifice while Ubik shows her and everyone present the truth of how her mother died: the Count's wife was discovered in a pagan orgy of her own accord while he was gone, and unable to kill her, he sacrificed her to the God Hand to escape his sorrow and become an Apostle. Theresia is nearly sacrificed, but the Count cannot bring himself to do it, and she witnesses him dragged off the to Abyss and, as the real world rematerializes around her, sobs over the human corpse of her father.

Theresia considers suicide and attempts it after being goaded by a dispassionate Guts, only for Puck to intervene. Nearly falling off the cliff, she is given a foothold by Guts, who brings her back onto the ledge. Regardless, she angrily blames him for killing her father and ruining her life, and swears she will have revenge on him one day. Guts cockily takes this in stride, reminding her that she now has a reason to continue living, though deep down he is saddened by what he has turned Theresia into.