If you want you character to wear casual clothing and still stand out why not try an accessory of some sort. What about a facial piercing it can make someone stand out. The person can have some sort of clothing fetish, maybe they like boots more than the average person or criter hats. Maybe a medallion with significant meaning or even give them an interesting tattoo or scar.
Give your character a particular style. If he's a rebel dress him/her in an edgy street style- something punk, scene, gothic. If you want to emphasis intelligence try sweater vests, ties, collared shirts, glasses? Innocent girls are usually dressed in skirts and dresses, adorned with cute accessories like bows and hair clips.
even the type of jacket can say something about a character- like a hoodie can indicate a person is more laid back verses a person in a peticoat or a trenchcoat.