Hiromi is the best friend of Haru Yoshioka, the protagonist of the film, and serves as a key figure in the ordinary human world that Haru temporarily leaves behind. She is a teenager with brown eyes and brown, neck-length hair, often styled with distinctive cheek curls, and is typically seen in her school uniform with a bow.

Hiromi’s personality is designed to be a clear contrast to that of her friend Haru. While Haru is often portrayed as awkward, clumsy, and somewhat uncertain, Hiromi is more outspoken, confident, and outgoing. She has no hesitation in voicing her opinions directly, whether expressing disdain for a careless truck driver or calling out a cat for walking into dangerous traffic. This grounded and sometimes blunt nature makes her a stabilizing presence in Haru’s life. In their school environment, she appears to have her life more planned out than her friend does.

Within the story, Hiromi’s primary role is that of a supportive confidante. She is with Haru at the film’s inciting incident, witnessing the near-accident of a cat carrying a package, though she does not believe Haru’s subsequent claim that the cat spoke to her. Her disbelief represents a normal, rational reaction to the extraordinary events unfolding, which helps to highlight the bizarre nature of Haru's adventure in the Cat Kingdom. Beyond this event, she remains a constant in Haru’s school life.

Regarding key relationships, her most significant bond is with Haru, whom she accompanies to and from school. Her life also includes a romantic interest in a classmate named Tsuge. Her affection for Tsuge is a notable motivation for a small but character-revealing action; she asks Haru to take over her cleaning duties after school so that she can attend his table tennis game, and she is visibly cheerful when he wins. She lives in an apartment, and her parents are never seen or mentioned within the film.

Hiromi does not undergo a significant personal transformation or development arc, as her role is to provide a stable anchor to the human world. She serves as the primary example of daily life and routine that Haru risks losing as she becomes more entangled in the affairs of the Cat Kingdom. Her notable abilities are not supernatural or extraordinary; rather, they are social and personal. Her key strength is her confident, outspoken personality and her ability to act as a clear-headed and pragmatic friend.