I know it's rather early to be speculating about these kind of things, i just think she will enter the house during a twist or task and become an actual HM. The reason being she is a lot younger than Joe and pretty hot (from what i've seen) and BB will hope her being there will cause friction between Joe and all the younger cocksure lads in there?!
Notice in his VT (assuming that is his Girlfriend printed on his T-Shirt) him wearing a T-Shirt with her baring her bum. I mean if you allow your BF to wear a picture of you with your ass out on a T-Shirt i imagine she doesn't mind flaunting her stuff. It would be interesting to see how she acts in there and how the other men are around her knowing she is in a relationship with Joe.
It's way out there i know i just have a feeling she'll feature sometime this series!
Joes VT - His GF appears around the 0.22 mark on his T-Shirt, as he is talking about her