Help for a book please? Read description :)?

I am writing a fantasy book, and there is a character in which is a warrior. A girl meets him, and the boy is nice to her, a charmer basically. She is rude to him, not wanting to be close to him, eventually they become close. Anyways, I want the boy to have a bad past, or a secret she does not know of (And will be very upset about when she finds out) I would love for you to give me ways of coming up with ideas for this because I do not want to come off as taking people's ideas D: Yet, if you would like to give me a straight forward answer, I'll take it. Thank you. :)