The Great Triangle! Someone call Pythagoras, quick!
Ahem. Anyway, Hikari--as she stood in the original game, I might make some adjustments--is the daughter of succubus and a normal human, though she's not aware of that fact; as far as she knows, she's just the product of what could be any other single-parent family in Japan, if obviously of mixed ancestry.
Hikari was a very talented gymnast when she was younger, and there was even talk she might be fit for Japan's Olympic hopefuls, but she hit puberty early, and hit it hard; her figure is now quite a bit too exaggerated in certain ways for her to be seriously competitive as a gymnast, though she's still a star at the local/regional level. Aside from gymnastics, she participates in a few other extracurricular activities: she's in Fashion Club (which many students now cheekily refer to as 'Cosplay Club' because of her many costume projects on the sewing machines) and a card-carrying member of Anime/Manga Club, though as a mostly-male club she doesn't seem to have much in common with the other members, given her obsession with Shoujo Manga.
Hikari isn't especially bright and is extremely naive; as one would expect, her grades are fairly mediocre, and low enough to be conspicuous by Tokyo standards. This, combined with her obvious signs of mixed ancestry (hair color and style) and buxom figure, make her a target of quite a bit of bullying by other girls. There are still a few who like her due to her infectiously cheerful and infallibly polite manner, but she's definitely not part of the 'in' cliques. Though she's somewhat well-liked by the boys (for obvious reasons), she has a bad reputation as being 'Spacegirl'; essentially, she's impossible to get a date with because she's completely oblivious to the fact boys are trying to come onto her. There's a running game in some of the junior and senior class crowds as to how blatant a come-on someone can make toward her without her realizing it.
As she is a virgin, her half-succubus nature is currently dormant. However, once she loses her virginity it will activate, though for the most part it remains latent and uncontrolled: she projects an aura of lust and desire. This aura is normally fairly mild, causing things people say to seem more suggestive, for instance, and making arousal more sudden and more evident... but not actually causing people to become attracted to those they normally wouldn't be, or to do things they wouldn't do. However, the longer she goes without sex, the more desperate this aura becomes: after a few days, it starts to insinuate perverse ideas into anyone nearby, and can cause those with weak-wills to be overcome by a voracious (and rapacious) sexual appetite. After a week, the effect elevates to a near-compulsion driving anyone in close proximity into a frenzy of lust--though, for it to reach this stage without her being isolated for an entire week is unlikely, for obvious reasons.
This aura is completely out of her hands; she does not realize it exists, and wouldn't be able to activate or deactivate it even if she did. The aura, notably, does not affect her, and is likely ineffective against other succubi and incubi (not that they need the encouragement).