Kasumi Hiroyama is a character who appears as a student at the high school Seishu Handa attends. She serves as the school librarian, or at least performs library duty during lunch breaks and after classes, as indicated by her uniform, which matches the school’s style although her skirt is notably longer than those worn by other female students. Her primary role in the story revolves around her strong romantic interest in Handa. To express her feelings, she orders numerous books on calligraphy specifically for him, hoping to appeal to his interests, and she also hides romance novels among them.

Her personality is initially defined by this quiet, somewhat indirect infatuation. She prefers to express her interest through actions taken in the library rather than through direct confrontation or confession. Later in the series, her behavior changes dramatically after she is subdued by another character known as Eraser. Following this event, she becomes subservient to Eraser and takes on a more covert and protective role, often lurking in the shadows to prevent other girls from approaching or bothering Handa. This shift turns her from a passive admirer into an active, if hidden, guardian who works to control Handa’s social interactions from behind the scenes. Her key relationships are therefore defined by her unrequited crush on Handa and her enforced servitude to Eraser. No notable physical or special abilities are highlighted for this character beyond her resourcefulness as a librarian and her dedication to her new, shadowy duties.