Rumi Nishihara is a high school student featuring ochre hair and brown eyes, born on October 30. Outwardly presenting as a typical student, she conceals a fervent passion as a dedicated fujoshi, deeply invested in boys' love narratives. This enthusiasm drives her to devote substantial financial resources to acquiring BL manga and fan merchandise while actively participating in numerous online fan circles. She possesses expert knowledge on sourcing rare BL items through both physical stores and digital platforms.

Generally reserved in social settings, especially around male peers, Rumi sheds her shyness when discussing BL topics. This shift enables her friendship with fellow student Ryo Sakaguchi, who shares her interests. Their collaborative dynamic involves mutual assistance in navigating BL fandom activities, such as visiting specialty stores and attending events like Comiket, where Rumi's expertise helps Ryo overcome hesitations about purchasing materials.

Her engagement with BL extends into her family life; both her sister and mother share this appreciation, indicating a normalized household environment for the genre. The narrative subtly hints at potential unspoken feelings Rumi may harbor toward Ryo, though this element remains undeveloped. Across all supplementary materials, her character shows no substantive development or expanded background, maintaining a consistent role as a supportive figure within the central friend group alongside individuals of diverse orientations and BL-related interests.