Yumemi Hoshisato is the younger sister of Kana Hoshisato and the daughter of Mamoru and Maeve Hoshisato, making her the niece of Mariah Grene. This places her within the central Hoshisato household, connected to magical activities and mentorship.

She presents as a petite young girl with notably long light blond hair and blue eyes. Her signature attire is an extra-long lavender outfit featuring pink stripes around the sleeves, complete with a bell and a tail; a red ribbon is tied near the end of this tail, creating a distinctive look.

Yumemi typically displays an apathetic demeanor, showing minimal emotional expression and rarely smiling. A documented exception occurs when Makoto Shiranui demonstrates his magic, prompting one of her few visible smiles and indicating a specific responsiveness to magical displays despite her usual reserve.

Her name carries meaning: "Yumemi" combines "dream, vision" (夢) with "not yet, still" (未), while "Hoshisato" derives from "star" (星) and "village" (里), reflecting potential thematic elements.

Identified as a primary school student, her involvement stems from her presence in the Hoshisato household and interactions with central characters like Makoto and Kana.