Like Koji said, the PC doesn't like to have non-volunteers live with volunteers; in fact, in some countries I believe it can be grounds for separation. While I was serving, we were not allowed to have visitors at all for the first and last three months of serving, not including training which was an additional 2.5 months at the beginning. Violating this was grounds for Administrative Separation (aka, being fired).
That being said, we did have one volunteer who had his girlfriend (now wife) live with him for most of the second year we were there. He had to get PC and his site's counterpart's permission. PC will not provide any financial, medical, or any other type of assistance for that person, which should be a big consideration.
Long story short, if it's the same as when I was in, she wouldn't be able to join you until month 6, would have to leave 3 months before you close service, and would have no support from PC in case of emergency.
Also, PC now allows non-married couples to serve, so that might be something to look into if that's an option.
Again, all based on my experience 10 years ago...so it may be different now.