Being a cat psychologist could be good money...
I'm by no means a pro. I just happen to have two Sealpoint Siamese kittens (soon 10 months old) and I have not noticed this behaviour in either of them. But they can play as much as they like with each other. Siamese HATES to be alone, even for 2 minutes. They get VERY upset if you leave them. So... what I'm thinking is that it might be that this little guy might be depressed and yes, he can also be jealous of your girlfriend, because you're not home with him during the days and then when she shows up, you pay her attention instead of him.
So maybe you could pay more attention to what this guy need. Does he need more playtime with you, so he's more willing to share you with your girlfriend? Or do you might find a way to keep him company during the days, so he doesn't get so obsessive when he finally has his human at home? Maybe save a kitten from a shelter to keep him company?
Also... involve your girlfriend in this guys life, so he might not feel that she's a threat but part of HIS family. Have her treat him some goodies, play with him, etc. Let him know that she cares about him.
I don't think there's some single solutions that will resolve the problem right away, but if you can find out WHY he's acting like he does, then it'll be much easier to find a longterm solution in the end, which lets you enjoy your cat and your girlfriend, plus that your cat and your girlfriend are on good terms.