Bruiser by Neal Shusterman

There's a reason why Brewster can't have friends—why he can’t care about too many people. Because when he cares about you, things start to happen. Impossible things that can’t be explained. I know, because they’re happening to me. When BrontË starts dating Brewster “Bruiser” Rawlins—the guy voted Most Likely to Get the Death Penalty—her twin brother, Tennyson, isn’t surprised. But then strange things begin to occur. Tennyson and BrontË’s scrapes heal unnaturally fast, and cuts disappear before their eyes. What at first seems like their good fortune turns out to be more than they bargained for . . . much more.