Defiance (Strange Angels, #4)

Now that sixteen-year-old Dru's worst fears have come true and Sergej has kidnapped her best friend Graves, she'll have to go on a suicidal rescue mission to bring him back in one piece.

That is, if she can put all of Christophe's training to good use, defeat her mother's traitor, Anna, once and for all, and manage to survive another day...