Spike must have mentioned her being a slayer, but she didn't hide it, hell she might have said it with the people she spoke with, but she nodded her head, "I am," she told him, "but" she added, "I only slay when an innocent is hurt, threatened, or on the verge of being hurt. It's part of who I am," she wasn't one to explain things properly.
"Why?" she countered back as she cocked her head to the side to look at him,