Ritsuko exists within a medieval European-inspired setting during episodes 3-4. Orphaned young, she and a boy named Gargantua were cared for by Jill, an incarnation of the cosmic traveler Eve, who assisted at their orphanage. A pivotal confrontation arose when Gargantua, consumed by jealousy over Jill's affection, accidentally stabbed her. Instead of dying, Jill dissolved into green energy ("souma"), drenching both children. This event granted Ritsuko and Gargantua immortality, halting their aging upon reaching physical maturity.

Traumatized, Ritsuko and Gargantua separated. Ritsuko developed healing abilities and became a traveling miracle worker, aging only a century physically while wandering the land. She carried small portraits of Jill and Gargantua in a locket, harboring deep unresolved feelings about their shared past. Her appearance evolved to include distinctive attire resembling BDSM gear and a snake tattoo near her pelvis.

Approximately 100 years later, Ritsuko arrived at a castle to heal the ailing Princess Mariel and was appointed court alchemist. There, she encountered Gargantua, now a practitioner of dark magic obsessed with locating Eve. Gargantua kidnapped Mariel, intending her as a sacrifice to the librarian Lilith in exchange for Eve's location. Ritsuko intervened too late to stop the ritual, resulting in Mariel's subsequent suicide due to feeling tainted by Gargantua's darkness. Authorities framed Ritsuko for the kidnapping, leading to her imprisonment.

External intervention by Eve and the fox spirit Tamamonomae ended Ritsuko's imprisonment and prompted a change in Gargantua. He returned to the dungeon and freed her. The two immortals departed together for the realm Lilith had previously granted Gargantua, implying reconciliation and a shared future.

In the original visual novel, Ritsuko utilized sexual magic, and a subplot involved her future self creating a clone. This clone traveled across dimensions with two children, though these specific elements were not adapted into the anime.