Otomi Tachibana, younger sister to Souichiro Tachibana and stepsister to Touma Tachibana, is only one year their junior. After her biological father Keiichi Sawai's early passing left her with no significant memories of him, her mother Mayumi Sawai remarried Eisuke Tachibana. This blended family includes Eisuke’s son Touma and their dog Punch.

Initially developing a shy, reserved demeanor and intense fear of strangers following her father’s death, Otomi gradually overcame these challenges through Touma’s consistent companionship and outgoing confidence. She maintains deep reliance on him for emotional support, often prioritizing Touma over blood-related Souichiro—evidenced by actions like preparing exclusive versions of food solely for him.

As a first-year Meisei Middle School student, Otomi plays flute in the music club’s brass band. Her musical skill and appearance fuel popularity among peers, attracting romantic interest from several boys, including classmate Ichiban Natsuno.

Her bond with Touma is uniquely special: she serves as his "first fan," convinced he "shines" while pitching—a connection noticeable even to Souichiro. Conversely, Souichiro acts as an overly protective older brother, frequently insisting Touma accompany Otomi if she might be alone.

Beyond family, Otomi directly asked Haruka Ohyama which brother was her romantic type, reacting with surprise to Haruka’s evasive answer. Her leisure activities, like bowling, occasionally prompt her brothers to speculate about her social life.