An androgynous high school student serving in the school's Host Club.
Haruhi was born to a lawyer mother and a drag queen father. Her mother, who she idolized, died when Haruhi was young, leaving her to be raised by her father. Her father claims to be bisexual but refuses to be with any woman but Haruhi's mother, and is therefore in effect gay, although we never see him with any other men. At some point, Haruhi decided that she wanted to be a lawyer like her mother. In order to do this she wanted to go to a top school, but her father can't afford it. As such, she works very hard in school and earns herself a scholarship to Ouran High School, a top-notch school where only extremely wealthy students attend.Shortly before school starts, she gets gum in her hair and cuts it short like a boy's, albeit poorly so that it sticks out funny. She also loses her contact lens, so she wears large glasses that were her grandfathers. She can't afford the school uniform, so she wears a sweater and slacks. Within a few days of her entry at the school (this timeline is unclear), she wanders around trying to find a quiet place to study and instead comes upon the Host Club. The members assume at first glance that she's a boy. While in the clubroom she accidentally breaks a very expensive vase that was going to go on auction to fundraise the club's activities. She is forced to become the club's gopher in order to pay for the vase, but before too long, the Host Club's president Tamaki notices Haruhi's very expressive eyes, hidden by her glasses.
The club members perform a makeover on Haruhi and turn her into a very attractive young man. They then try her out as a host and find that she is a perfect accent to their own personas: she is perfectly honest, sweet, and endearing. Unfortunately, the attention that the club members pay to her-- particularly Tamaki --causes one client to get jealous and she harasses Haruhi by tossing her backpack into a fountain. Haruhi gets her clothes wet trying to get her things back, and another host-- Kyouya Ootori, the most intelligent member, who has long since figured out Haruhi's gender, along with all the other members except Tamaki --offers her a change of clothes. When she leaves the changing room, she's wearing the school's uniform for girls, and only at this point does Tamaki realize that she's a girl.
The club members decide to allow the rest of the school to continue thinking that Haruhi is a boy, since otherwise it would be difficult to keep her as a member.
Haruhi is keenly intelligent and has excellent insight into the feelings of others, except when they have to do with her-- she tends to be clueless as to the growing feelings of nearly all of the hosts around her. This ability allows her to help the others sort through the difficulties they have as each of them grows up and works out who they are and who they want to be. However, it does not extend to herself, as certain aspects of life leave Haruhi perplexed. Due to her father's unusual stance on matters of gender, Haruhi claims she has never really worried much about acting out a particular gender role, which is why she was perfectly content to allow the host club members to think she was a boy. She is not particularly concerned with the matter and the hiding of Haruhi's female-hood is not a major part of the series' plot.
Her simplicity and lack of any interest in melodrama set Haruhi apart from your average shoujo heroine. She likes seafood, especially fatty tuna (ootoro) though she has never tried it. She has a severe phobia of thunder and lightning.