Haruhi Fujioka is the central character of the story, a first-year student at the prestigious Ouran Academy who enters the school on a scholarship due to her exceptional academic performance. Her background stands in stark contrast to the wealthy elite who make up the majority of the student body, as she comes from a modest, middle-class family. She lives with her father, Ryouji Fujioka, who works as a cross-dressing entertainer under the stage name Ranka at a bar. Her mother, Kotoko Fujioka, a lawyer, passed away from an illness when Haruhi was very young, an event that profoundly shaped her character and aspirations. As an only child, she assumed many household duties early on, fostering a strong sense of independence and practicality.

In terms of personality, Haruhi is defined by her straightforward, level-headed, and remarkably practical nature. Unlike her peers, she is not easily impressed by wealth, status, or the flamboyant charms of the Host Club members, often reacting with dry exasperation or nervous confusion rather than infatuation. She has a firm belief that a person's gender should not define their personality or how they are treated, a philosophy influenced by her upbringing with her father. This leads her to be dismissive of conventional social pretensions, including those tied to appearance and status. She is habitually unflappable in most situations, with a notable exception being her intense fear of thunderstorms, which can leave her paralyzed. Despite her often apathetic or grumbling exterior when dragged into the Host Club's schemes, she genuinely cares for her fellow members. Her love for food, particularly fatty tuna, is a well-known trait that is frequently used as a bribe to secure her cooperation.

Haruhi's role in the story begins after she accidentally breaks an expensive vase belonging to the Host Club, valued at eight million yen. To repay this debt, she is compelled to join the club as a host. Due to her short hair, androgynous appearance, and choice of the more practical male uniform, she is initially mistaken for a male student by the club's clients and most of its members. She agrees to continue posing as a male host to work off her debt, becoming the club's only commoner member. Her hosting style is known as the natural type, as her candid, no-nonsense, and sincere interactions with clients prove to be a refreshing contrast to the more theatrical personas of the other hosts. Her primary motivation is to become a lawyer, following in her mother's footsteps, and she prioritizes her studies above all else, viewing her time in the Host Club as a necessary burden for debt repayment.

The key relationships Haruhi forms are central to her character development. Her dynamic with Tamaki Suoh, the club's president, begins with his over-the-top declarations of paternal affection, but slowly evolves into a mutual romantic love that culminates in them becoming a couple and eventually marrying in the manga's conclusion. She shares a unique bond with the Hitachiin twins, Hikaru and Kaoru, as she is the first person outside their world who can easily tell them apart, which encourages their individual growth. With Kyoya Ootori, the vice-president, she shares a pragmatic alliance based on mutual respect and her debt, though he is one of the first to discover her secret. The older members, Mitsukuni Haninozuka and Takashi Morinozuka, treat her with a sibling-like protectiveness. Outside the club, her most important relationship is with her father, Ranka, whose supportive and unconventional nature she deeply appreciates.

Throughout the series, Haruhi undergoes significant development. Initially isolated and determined to rely solely on herself, her interactions with the Host Club gradually teach her about friendship, trust, and the value of allowing others to support her. She moves from seeing the club members as eccentric burdens to considering them her dearest friends. Her emotional journey includes becoming more aware of her own feelings, particularly her romantic love for Tamaki, a realization she is initially oblivious to. Her perspective expands as she learns to enjoy the present moment and the connections she has made, rather than focusing solely on her long-term goal of becoming a lawyer. Among her notable abilities, Haruhi is exceptionally intelligent and consistently ranks at the top of her class. She possesses keen observational skills and a natural empathy that allows her to understand people's true feelings, which makes her an effective host. While she is not athletically inclined and admits to being a poor singer and dancer, her resourcefulness, honesty, and unshakable moral compass are her greatest strengths. Physically, she is of short stature and has a slender, boyish figure that contributes to her androgynous appearance.