Buffy didn't know who the guy was, but he was either an idiot or someone who actually knew what was going on. He reminded her a little bit of Pike in a way. He had the same sort of disaffected but sexy loner thing going on and he seemed nonplussed by the very obvious fangs and bumpy forehead look.
"A knife's not going to do it," she responded, kicking one of the vampires and then stabbing him in the heart with a stake.