I think I'm going to go with a teen living in the heart of NYC with a group of similar-minded teens, where the whole group has little/no money and basically steals what they need to survive, kind of a bit of self-formed thief outfit. Over time, he starts to notice that he's a bit stealthier than his companions, quicker at breaking in, etc. Add to that the slow growth of extra hair over the whole body (turning into fur), as well as nails slowly lengthening into claws. Basically a slow transition from human to a catfolk rogue, or cat burglar, if you will.