Is this a constant issue or just sometimes as you say in your post? If it's just sometimes, don't worry about it. It's just words. I mean, are they just occasional negative comments, fairly benign, just kind of made in passing? Or are they persistent, really mean, hateful? Makes a difference in my book. It isn't uncommon for a pet to be close to the owner and no one else is all that attached to the pet. They might even say occasional negative things about the pet, particularly if they aren't pet people. As long as it isn't abusive or constant in some way, you can kind of just let her gripe occasionally and just blow it off. Your cat doesn't care if you defend it verbally to your girlfriend (I'm pretty certain about that). My guess is if you just sort of blow off the comments, the comments might subside and girlfriend and cat will learn to coexist, perhaps not affectionately but not with hostility either.