Princess Mariel is the princess of a medieval European-style fantasy kingdom. She becomes critically ill, and the immortal magic user Ritsuko is brought to the castle to heal her. After her successful recovery, the two form a close bond, and Mariel is often escorted through the village by Ritsuko.

Gargantua, an immortal individual obsessed with locating Eve due to a childhood trauma involving her power, kidnaps Princess Mariel. He intends to sacrifice her to Lilith, the guardian of the Great Library, in exchange for information on Eve's whereabouts. Though Ritsuko intervenes and halts the ritual, Mariel is left tainted by Gargantua's dark magic. Believing herself corrupted by this darkness, she takes her own life by poison.

Her narrative intertwines with themes of sacrifice, unintended consequences, and tragedy resulting from the actions of immortal beings.