Nanami Hiromachi is a second-year student at Tsukinomori Girls' Academy, sharing a class with bandmate Tsukushi Futaba. She serves as the bassist for the band Morfonica and designs their promotional posters. Nanami lives with her parents in a large house featuring an atelier, which doubles as Morfonica's practice space. Her mother is a painter and her father a sculptor, both recognized artists known for their eccentric and quirky behavior. This led Nanami to avoid letting others interact with her parents. Influenced by her grandmother's superstitions, such as the belief that standing in the center of a photograph steals one's soul, Nanami herself developed superstitious tendencies.

Growing up with highly artistic yet unconventional parents fostered Nanami's strong desire to be perceived as normal, believing this essential for making and keeping friends. Despite inheriting her parents' intellect, talent, and artistic abilities—which attracted high expectations and peer jealousy—she experienced significant isolation during middle school, especially after winning an art competition. This loneliness prompted her to conceal her true capabilities. She joined Morfonica primarily because girls' bands were trending, viewing it as a pathway to the ordinary high school life she craved. Nanami is also a member of Tsukinomori's Horror Research Club, where activities include watching horror films.

Nanami has pink eyes and peach-pink hair styled in twin tails secured with yellow ribbons or black hair ties. A black-and-white cat hairpin accents her bangs. Her initial outfit consists of a white long-sleeved shirt with a detailed collar and puffy sleeves, layered under a pale pink pinafore dress, accessorized with a belt and a small cross necklace. Seasonal variations include a cream-white turtleneck sweater with rose-colored sleeves and a multi-colored pleated skirt for winter, and a short-sleeved white T-shirt paired with a pale green knee-length pinafore dress for summer.

Personality-wise, Nanami is self-conscious, constantly questioning if her actions or words align with societal norms to avoid standing out. She hides her true self to blend in, downplaying accomplishments with phrases like "it was just a coincidence." Despite this facade, she excels academically and possesses exceptional artistic skills, enabling her to learn tasks quickly through online research—such as song arrangement or crafting accessories for the band. Ironically, her pursuit of normalcy sometimes leads to unconventional achievements, like deliberately earning average scores on all tests. She struggles with interpersonal conflict resolution due to limited friendship experiences, once resorting to internet searches for guidance.

Over time, Nanami experiences subtle development through her involvement with Morfonica and other activities. She takes a part-time job at a family restaurant and later earns a promotion to manager. Her engagement with the band and gradual self-acceptance mark an evolution from hiding her talents toward beginning to embrace her uniqueness while still navigating her desire for normalcy. Her hobbies include collecting toy prizes from commercial products, enjoying spicy and unique foods, reading the occult magazine "Supernatural Encounters" with Hina Hikawa, and maintaining a tank of artemia.

Her character appears in various BanG Dream! projects, including the mobile game Girls Band Party!, the anime series Garupa☆Pico: Oomori, and the two-episode original anime Morfonication.