Erika Ainsworth appears as a member of the Ainsworth family within their parallel world. Her origins reveal she is not human, but Pandora, the first human crafted by the Twelve Olympians in Greek mythology roughly 6,000 years before the narrative's events. In this reality, Pandora's Box remained sealed, trapping her "death" inside and granting her immortality. Centuries of persecution followed as humans, discovering her nature, repeatedly tried to kill her. This inflicted profound suffering and cultivated her deep-seated hatred for humanity. Darius Ainsworth found her approximately 1,000 years ago; she has lived with the family since, adopting numerous aliases over generations, with "Erika Ainsworth" as her current identity. Erika typically displays a cheerful, innocent demeanor, often seeming oblivious. This childlike facade conceals a cunning, manipulative nature, earning Illyasviel von Einzbern's description as a "twisted child" whose apparent innocence lures others into underestimating her danger. Her actions, however, stem primarily from a fear of loneliness, driving her to extreme measures to avoid abandonment. She exhibits genuine affection for Miyu Edelfelt, whom she views as an older sister, and becomes visibly distraught when Miyu declares they cannot be friends due to opposing sides. Erika also possesses a nuanced grasp of Darius Ainsworth's expectations, recognizing he would disapprove of bringing Illya to the castle unless rendered powerless; this understanding prompts her to transfer Illya's soul into a stuffed animal after an initial brainwashing attempt fails. Her reawakening as Pandora triggers a dramatic personality shift, unveiling her true mindset: cold, disillusioned, and consumed by a desire for death after enduring millennia of suffering. This transformation occurs abruptly when triggered by trauma reminders, resulting in a demeanor mirroring Julian Ainsworth's detached resolve. Pandora explicitly states her sole wish is to die and be released from her duty, showing no regard for humanity's survival. Erika's actions significantly drive the plot. She infiltrates Illya's hideout by exploiting Illya's desire for a sibling relationship, later orchestrating Illya's kidnapping to appease Miyu. Failed attempts to subdue Illya through brainwashing and soul transference lead to Darius's punishment. A critical turning point arrives when Miyu rejects their friendship, triggering Erika's emotional breakdown coinciding with her exposure to a mysterious "mud cube." This exposure physically transforms her into a slightly older form and fully awakens her Pandora persona. In this state, she confronts Illya, revealing her true identity and expressing her death wish. Later, she assists Darius in unleashing Pandora's Box over Fuyuki City to end human civilization. When the box's opening halts due to Illya's survival, Pandora becomes trapped in suspended animation beneath the mud. After Illya resets the timeline, Pandora's interactions with Darius grow antagonistic; she openly mocks his failures during the final battle, taunting him about possessing descendants and his dwindling options once Julian turns against him. Her abilities center on Pandora's Box, a Noble Phantasm functioning as both a weapon and the source of her immortality. The box contains "a world's worth of ruined possibilities," manifesting as corruptive mud capable of generating hostile entities. Exposure to this mud transforms Erika's body, making her slightly shorter yet appearing older. The box cannot be fully emptied until all human life is extinct. As its "observer," Pandora is excluded from this definition, allowing her to persist even after the box's activation.