Chihiro Onizuka is first introduced as an elementary school classmate of the protagonists Akira Ono and Haruo Yaguchi. Her family name, Onizuka, carries connotations meaning "demon mound," a detail sometimes noted in relation to her unconventional appearance. Throughout her school years, she remains part of Doi Genta's social circle and continues to attend the same middle school as Yaguchi, where she is in the same class with him during their second year. Her high school years are spent at the same all-girls school as Koharu Hidaka. Physically, she is typically depicted with pink hair, though some sources describe her hair as dark pink or blonde, and she has black eyes. As an adult in the sequel series Hi Score Girl DASH, she is shown to have become slimmer and more slender than she was in her teens.

Chihiro is well-known for her bold, crass, and uninhibited personality. She exhibits a remarkably high level of sexual curiosity for her age, often demonstrated through outrageous comments and behaviors, and she appears to lack self-awareness regarding her own appearance. Despite her crude demeanor, she is also portrayed as a loyal and good-hearted friend. Her romantic focus is directed almost entirely at her classmate Kotaro Miyao, for whom she harbors an unrequited crush. Her attempts to win his affection are notably creative and persistent, such as the time she gave him a Valentine's Day chocolate molded in the shape of her own face. Miyao, however, consistently and skillfully dodges all of her advances. Outside of her romantic pursuits, one of her most remarkable abilities is her talent for makeup. In a bonus manga chapter, her makeup skills are showcased as being so transformative that even Miyao fails to recognize her after she has applied it.

Chihiro's role in the story is primarily that of a supporting character, a colorful and memorable part of the ensemble that populates the main characters' school life. Her key relationships include her long-standing friendship with Koharu Hidaka, which continues into their shared high school years, and her more distant but notable connection to Haruo Yaguchi, whose gaming-centric world she observes from the periphery. The depth of her friendship with Hidaka becomes especially apparent in her development as an adult. By the time she is 28 years old and marrying, she has matured yet retained her essential lively nature. She is depicted as a smoker, sometimes using a pipe, and demonstrates a continued, selfless concern for her friend's happiness. In a significant gesture during her own wedding ceremony, she deliberately throws her bouquet to Koharu Hidaka, explicitly wishing for her friend's romantic future and well-being. This act serves as a testament to their enduring bond and highlights Chihiro's growth from a boy-crazed student to a thoughtful, if still eccentric, adult.