Hiromachi Nanami attends Tsukinomori Girls' Academy as a second-year student, sharing a class with bandmate Tsukushi Futaba. She serves as Morfonica's bassist and designs their promotional posters. Nanami lives with her parents in a spacious house featuring an atelier, which also functions as Morfonica's practice space. Her mother, a painter, and father, a sculptor, are recognized artists known for eccentric behavior. To protect her social acceptance, Nanami prevents others from interacting with them. Influenced by her grandmother's superstitions—like the belief that standing in a photograph's center steals one's soul—Nanami adopted similar superstitious tendencies.

Her parents' unconventional reputation and high expectations stemming from inherited intellect and artistic abilities fueled Nanami's intense desire to be perceived as normal. She believed conformity was essential for making and keeping friends. Yet her talents often provoked jealousy, particularly after winning a middle school art contest led to isolation. This experience caused her to conceal her genius-level capabilities, habitually downplaying achievements with phrases like "it was just a coincidence" or "this is nothing special." She joined Morfonica during the peak of the girls' band trend, viewing it as a pathway to an ordinary high school life.

Nanami possesses peach-pink hair styled in twin tails secured by thin yellow ribbons or black hair ties, pink eyes, and a black-and-white cat hairpin holding her bangs. Her initial outfit comprises a white long-sleeved blouse with a detailed collar, a pale pink pinafore dress, a belt, and a small cross necklace. Seasonal variations include a cream-white turtleneck sweater with rose-colored sleeves, a multi-colored pleated skirt, and a key necklace in winter; and a short-sleeved white T-shirt with a pale green pinafore dress in summer. She stands 157 cm tall.

Her personality centers on an obsession with normalcy. Nanami constantly questions if her actions or words align with social norms, actively masking her true self to avoid standing out. Lacking prior friendships left her socially inexperienced; she once searched online for guidance on resolving arguments. Despite efforts to appear average, her innate abilities surface effortlessly—she excels academically, creates band accessories, and arranges songs after minimal internet research. Ironically, this pursuit of mediocrity sometimes yields unconventional outcomes, like meticulously achieving perfectly average test scores, a feat requiring exceptional precision.

Beyond Morfonica, she belongs to Tsukinomori's Horror Research Club, engaging in activities like watching horror films. This interest aligns with her enjoyment of the occult magazine "Supernatural Encounters," which she reads alongside Hina Hikawa of Pastel✽Palettes. Her hobbies include collecting toy prizes from commercial products and maintaining a tank of artemia. She favors spicy and unique foods and has no specific dislikes.

Over time, Nanami experiences subtle development. She begins working part-time at a family restaurant during the "Iris of Secrets and Youth" event and later advances to a managerial position, as noted in "Let's Start a New Party!" Her involvement with Morfonica gradually fosters self-acceptance, allowing her to cautiously reveal more talents, such as contributing creatively to the band's arrangements and designs. This progression marks her shift from isolation toward forging genuine connections within her social and musical circles.