Sayomi Hanawa is Kaoru Hanawa's older sister. She studies at an unnamed university outside Takehara, returning home during holidays or breaks from her unspecified part-time job. Her personality blends lighthearted enthusiasm with occasional clumsiness, presenting a somewhat childlike demeanor that contrasts with her age. This inconsistency between her behavior and adult responsibilities sometimes strains her relationship with Kaoru, who finds her antics exhausting.

Her primary passion involves discovering small, scenic, and secluded spots in the countryside, often organizing lengthy hikes to reach them. While she shares an interest in photography, this gradually becomes secondary to the act of exploration itself. She frequently initiates group outings, inviting Kaoru, Fuu Sawatari, and their friends on these excursions. These trips typically feature extended walks and instill apprehension among participants due to her notoriously poor driving skills, which evoke visible fear in passengers.

Sayomi plays a recurring role in group dynamics through these organized adventures. She assists others in locating meaningful places, such as helping Fuu identify a site connected to a childhood drawing related to Fuu's father. Her influence also extends to coining Fuu's nickname "Potte," based on the sound Fuu made when falling during a childhood visit to Takehara.

Her university life, career aspirations, and long-term goals remain unexplored. Her character, background, hobbies, and relationships show no significant development across the series, OVAs, or films. Her interactions consistently emphasize her role as an enthusiastic yet occasionally overbearing presence within the group.