From The Grey House:
She looked out the window at the passing trees. “I think part of me knew I wasn’t supposed to be a Slayer; I never told Zechariah about my training. I let him think I was a researcher. Every so often I would track a vampire on my own to practice and train. Every so often I would offer them their life in exchange for a fight. If they could kill me, they’d be free.”
“A little dangerous, don’t you think?” His voice told her what kind of fool he thought she was.
“I’m still here aren’t I?” There was pride in her voice.