Harumi Taniguchi is a major supporting character in Citrus and serves as the first friend that Yuzu Aihara makes after transferring to the strict all-girls academy. Known to Yuzu by the affectionate nickname Haruming, Harumi stands out among the student body for her easygoing and fashionable nature, which contrasts sharply with the school's conservative atmosphere. She has plum or dark auburn hair, usually styled with straight bangs and worn in low twin pigtails, and hazel eyes. At school, she maintains a neat appearance, but outside of its walls, she dresses in a much more daring, trendy style, reflecting her self-identification as a gyaru in disguise.

Harumi is characterized by a genuinely kind, humorous, and responsible personality. Unlike many of her peers who are preoccupied with the school's strict hierarchy, she is down-to-earth and becomes a reliable confidante for Yuzu, offering advice and support whenever problems arise. She is described as having a big sister-like aura and is very protective of those she cares about. Despite her cheerful exterior, Harumi has a tendency to hide her true feelings to avoid conflict or getting hurt. This defensive side of her personality is noted by the younger character Matsuri, who observes that Harumi often watches situations from a distance and maintains her gyaru persona as a form of self-protection.

Her role in the story is primarily that of a trusted friend and helper. She is Yuzu's anchor in an unfamiliar environment, often aiding her in navigating school rules and romantic troubles. One of her most notable and unique abilities is using her large bust as a hiding place for contraband items, such as her cellphone and makeup, a compartment she humorously nicknames Mineko's Room. She also has a stated fondness for history as a school subject.

Regarding key relationships, her bond with Yuzu is the most significant. Their friendship is built on a mutual understanding as both are relatively new to the academy's rigid culture. While she is unconditionally supportive of Yuzu's relationship with her stepsister Mei Aihara, the narrative subtly suggests, particularly through Matsuri's teasing, that Harumi might harbor romantic feelings for Yuzu that she chooses not to act upon. In one instance, she confesses to Yuzu only to quickly frame it as a declaration of platonic love, leaving the sincerity of her feelings ambiguous.

Harumi also has a complex dynamic with Matsuri Mizusawa. Initially wary of Matsuri's manipulative tendencies, Harumi becomes a frequent target of Matsuri's teasing and aggression. Over time, however, their relationship evolves into a close, bickering friendship. Harumi becomes one of the few people who sees past Matsuri's tough exterior and offers her genuine advice about self-acceptance. She shares a less prominent but friendly relationship with the student council vice president, Himeko Momokino, whom she delights in teasing, and she lives in fear of her own older sister, Mitsuko Taniguchi, a former student council president known for her stern demeanor.

Throughout the series, Harumi shows significant development primarily in her emotional openness. While she starts as someone who keeps a safe distance from drama to avoid personal pain, her friendship with Yuzu gradually encourages her to engage more deeply with others. In the sequel Citrus, she is shown actively reaching out to comfort and advise Matsuri, demonstrating a greater willingness to involve herself emotionally in her friends' lives and encourage them to embrace their imperfect selves, a lesson she has been learning herself.