You find Heather dying in a hospital in Santa Monica. You can save her by letting her drink some of your vampire blood, temporarily transferring your self-healing powers to her. If you do so, she later tracks you down and asks you to let her help you. She'll stay at your apartment and get you things.
The repeated drinking of your blood creates a master/slave bond with elements of addiction and even greatest affection. Eventually it becomes clear that keeping Heather around is going to destroy her and you can either send her away, breaking her heart but saving her life or decide to keep her as your slave (or companion... however you prefer to look at it).