Well, Erin's character highly depends on Voldemort, so it does need to have quite a lot to do with the books. But from enough of a different perspective, stories that happen side by side can have very little to do with each other. It depends entirely on what you want to do. Tell me more about this character idea you have, and we can run with it. It can be during the books, during the points that got ignored during the books (ie happening mostly in summer), or even after the books, depending entirely on what sort of game you're thinking of doing.

I have read all of the books, but I think books six and seven were actually submissions for ff.net that got published by mistake. >_> They read like bad fanfiction, and I'm thoroughly unimpressed with them. That said, if we pick up from book four with the TriWizard Tournament, we can write our own (better) ending to the HP saga if you don't mind doubling as the canon characters as well.