The Wood Sprite. Wallowa Lake Series, #2

Agreeing to a dubious pact offered by a paranormal creature propels shy sixteen-year-old Mia Chandler down a perilous path, unprepared and alone. She is challenged by terrifying events that require all her courage and intelligence to survive. Learning she has an unknown half-brother living in Oregon's beautiful Wallowa Mountains, Mia goes to meet him, but finds him and two of his close friends being stalked by a crazed killer.
Joining their struggle against this murderer, she feels an amazing preternatural ability arise within her when it is urgently needed.  This eerie power will transform the rest of Mia's life, as well as those of others.