Haruka Shiraishi is a main character in the story and a first-year middle school student. She has brown, shoulder-length hair and purple eyes, and is often seen in her school uniform with a bow. Haruka is best friends with her classmate Yako Jinguji, with whom she first encounters the protagonist, Nayu Hayama.

Haruka is notably more physically developed than many of her peers, which is a source of significant personal distress for her. She feels ashamed and embarrassed about having a larger bust, as it makes her the center of unwanted attention from male students. Her self-esteem suffered greatly in the past, specifically when she discovered a list created by boys that ranked her as number one in their school for breast size. This history of being ostracized in grade school for developing at a young age has led her to have trouble with boys, who tend to mock or objectify her because of her figure. She is generally a reserved and shy student, though others observe her to have a kind and caring personality.

Haruka comes from a family with three younger brothers and takes an active role in helping to raise them. This experience has given her a motherly nature and made her adept at taking care of others and sometimes scolding them, which Nayu and Yako often admire. She also appears to speak with a Kansai dialect.

Within the story, Haruka becomes a founding member of the Underwear Appreciation Society, the club Nayu creates to educate others about proper underwear. Her motivation for joining is largely to support her friends, though she remains shy about the club's activities. One of her key personal struggles involves learning to accept her body. Despite her discomfort, she agrees to act as a model for the club’s fitting sessions to help Nayu gain awareness for their cause, and is pleasantly surprised by the kind response she receives.

Haruka shares a close bond with Yako Jinguji, who acts as her best friend and fierce defender. Her relationship with Nayu is central to her development. She develops a strong emotional attachment to Nayu, which some characters interpret as possible romantic feelings, leading to speculation that she may be a lesbian. This attachment is tested late in the story when Nayu announces she will be transferring schools. Feeling betrayed by the news, Haruka briefly ends their friendship and refuses to speak to her. However, with Yako's encouragement, she ultimately reconciles with Nayu, apologizing and saying a proper goodbye before Nayu's departure.

A notable ability of Haruka's is her maternal skill set, stemming from her experience raising her younger brothers. Beyond that, she does not possess any special or unusual abilities within the context of the story. Her primary role is to represent a perspective of body insecurity and find a path toward self-acceptance through the support of the underwear club and her friends.