As a "girlfriend", she would not be eligible for the Annual Pass discount or any other membership perks. Most perks could be enjoyed by both of you when you (as the member) are present. But the AP perk is more rigidly structured to allow for only legal / blood relatives living at the same address as the member.
That said, if you put her on the contract and later part ways, she retains full legal ownership of the
DVC property. Doesn't matter if you actually paid for the points and/or dues. DVC contracts cannot be split up, nor could you remove her as an owner without her consent. You would be stuck with her as a joint owner until she agrees to relinquish her rights.
If she files for bankruptcy, the contract could be seized.
If you die, she has full ownership of the contract.
If you make a reservation, she could cancel it.
If you want to sell, she has to sign the paperwork and likewise agree to sell. (And could dictate her own terms, like wanting half of the proceeds.)
If she later marries someone else, the contract could be considered part of their community property.
You may be inclined to dismiss any/all of these possibilities, but any of them could make life VERY difficult for you down the road if something happens to the relationship.