Posi hails from the extraterrestrial realm Feather Star, serving as one of two feline-like guardians overseeing Yuu Morisawa during her year-long possession of magical powers. The name "Posi" comes directly from "positive," embodying an optimistic and encouraging nature that often provides a counterpoint to Nega's caution.

Posi offers guidance and support for Yuu's magical activities, sharing practical knowledge of magical mechanics and spells. This includes suggesting effective solutions, such as advising Yuu to wave her wand while reciting incantations to achieve successful transformations. While both guardians monitor Yuu's magic use, Posi generally shows greater leniency and trust, rarely restricting magical knowledge due to concerns about potential misuse.

Posi consistently demonstrates affection and loyalty toward Yuu, frequently advocating for her wishes and offering reassurance during difficult moments. This supportive approach sometimes creates friction with Nega, though their partnership remains cooperative in protecting Yuu's well-being. Distress surfaces when Yuu faces separation from the guardians or risks losing her powers, revealing Posi's deep investment in their mission.

Beyond direct guardianship, Posi fulfills a secondary narrative function by voicing episode titles, linking the character to the series' structural framework. The portrayal remains consistent across the television series and subsequent OVAs, with no documented evolution or divergent portrayals in spin-off media.