Su just wants to sit in a dark corner for a quiet drink when she spots a newborn vampire across the bar. He's confused, and he's starting to draw attention to himself. And he's hot.
So Su decides to give him a few pointers. Then she realizes that the reason this baby vamp is wandering around without a protective escort is that she killed his parent vamp earlier that night.
Now Su feels responsible. A newborn vampire is helpless until he regains his memories and learns how to act like a vampire. Su knows enough about vamps to be able to teach him that. But Su has her own problems. She doesn't have much of a memory, either, and while she's not a vampire, she's not exactly human.
Su doesn't know what she is, and she doesn't have a kindly stranger in a bar to tell her.