Is it possible to have a cat de-hissed?

You shouldn't live in the same neighborhood as a cat from what you are saying. If the cat is always hissing at you, you've obviously done something to her that you aren't telling us. tell your girlfriend it's me or the cat, that way when you show her your true colors, she can either get rid of the cat (in that case, she deserves you) or she'll pick the cat, and you can be done with the cat, and the girl.