Sweet sixteen - Scott Ciencin

Buffy's younger sister, Dawn, knows just how it feels to be different. so when she overhears some of her classmates making fun of a new girl, Arianna, she steps in, and the two become friends.
But when Buffy comes home one afternoon to find Dawn and Arianna hanging out, she's surprised -- she encountered Arianna one night out on patrol. A demon had been attacking her, but Arianna fought back on her own. It seemed to Buffy like Arianna was well able to look after herself. What's that about?
The more the gang gets to know Arianna, the more puzzled they become. She has super strength and accelerated healing power, just like a Slayer. But she also has a dark secret and an unusal past . . . and a dangerous legacy primed to reveal itself, just as soon as she turns sweet sixteen.