Nobody specifically asked for that before, but hey, if you want one, yeah sure, I'll allow it. Traits are not unbalanced anyways, so allowing a 3rd one in exchange for a flaw does not bother me.
Well, she's after his alter ego. Which is why when he introduced himself to her on the road he would have as himself and described himself as a vampire hunter. It's like how nobody knows Bruce Wayne is Batman except Ian's poorer and dumber.