[She actually manages to look slightly sheepish when she realizes what's being asked, glancing between Chell's hands and her own.]

Yeah, I know what you're thinking-- "But monsters aren't real," right? [She chuckles and gives an apologetic shrug, shaking her head.] Man-made ones are, as it turns out. Only a couple of us, though-- And don't worry, I'm not going to eat you or anything.

[Maybe a little grin will help to soften the blow a little bit? Eh, it's worth a shot.]

It's kind of a long story, but the short version, I guess, is just to say-- Well, werewolf.