Stranger, The (Vintage Point Horror) by Caroline B. Cooney

Till death do us part. 

From the first moment she sees him in class, Nicoletta is drawn to Jethro, fascinated by his dark, mysterious face. She is attracted yet repelled, interested but afraid - and unable to think of anything else. 

But she knows so little about him... until she follows him to a deserted cave on the edge of town, and learns the truth. 

Jethro isn't like the others at all. He has a terrifying secret - one that no one must ever found out.