Malorie Blackman 
Trust Me 

Seventeen-year-old Jayna wishes she could spend every second of the day with Andrew. He»s her first love, and he understands her like nobody else can. Jayna just wishes Andrew»s prejudiced family could accept her — but when they»re alone together, it hardly seems to matter.
But something strange has happened to Andrew. He looks . . . different. Pale and drawn, as if he hasn»t been outside for days, with mysteriously cold green eyes. He won»t go out in the sunlight, and he»s unnaturally fast and strong. And now he wants Jayna to join him.
Now she has to make the choice: to lose Andrew forever, or to be with him always — no matter the cost.