Slayer by Kiersten White

Nina and her twin sister, Artemis, are far from normal. It’s hard to be when you grow up at the Watcher’s Academy where teens are trained as guides for Slayers—girls gifted with supernatural strength to fight the forces of darkness. But while Nina’s mother is a prominent member of the Watcher’s Council, Nina has never embraced the violent Watcher lifestyle. Instead she follows her instincts to heal, carving out a place for herself as the school medic. Until the day Nina’s life changes forever.

Thanks to Buffy, the famous Slayer that Nina’s father died protecting, Nina is not only the newest Chosen One—she’s the last Slayer, ever. As Nina hones her skills with her Watcher-in-training, Leo, there’s plenty to keep her occupied: a monster fighting ring, a demon who eats happiness, a shadowy figure that keeps popping up in Nina’s dreams…

But it’s not until bodies start turning up that Nina’s new powers will truly be tested—because someone she loves might be next.