Haruna is the classmate of Rito Yuki, who both have a crush on one another. She somtimes becomes a guinea pig like Rito for Lala's inventions to which have awkward results.
In both anime and manga, Haruna is the one that Rito has always loved, but never had the guts to admit those feelings to. Whenever he worked up the courage to confess somthing would happen to prevent him from speaking (ie getting run over by a car/elephant/train).
The fun part, is that Haruna has always felt the same way, she was just too shy to step up and make the first move.
When Lala arrives and announces that Rito was engaged to her Haruna initially becomes supportive of Rito's apparent new 'fiance', even though she still loved Rito. Although she does eventually become genuine friends with Lala and admits to loveing Rito to Lala. Lala of course just smiles, laughs, and say's "that's great!!!"
Towards the end, when Rito admits his love to Lala and Haruna simultaneously, he goes first to Lala, confesses sucessfully. When he tries to do the same with Haruna... her bikini top snaps off and she turns away to hide herself. Rito had his eyes closed and doesn't notice, then shouting "I love you" to more than just Haruna.
Characterwise, Haruna fits in a usuall ecchi stereotype. The quite, shy girl that the male lead loves. She is also the most modest of the female cast. This still does not stop her from being the girl with the second most ecchi moments (second only to Lala). Haruna is scared to death of anything supernatural (ghosts) and will either faint or use Rito as a weapon to destroy the ghosts. This is rather ironic because in the manga one of her best friends happens to be a ghost that was given a body by Mikado.