The Warrior Heir by Cinda Williams Chima
Before he knew about the Roses, 16-year-old Jack lived an unremarkable life in the small Ohio town of Trinity. Only the medicine he has to take daily and the thick scar above his heart set him apart from the other high-schoolers.
At least, until one day Jack forgets his 'medicine'. Suddenly, he is stronger, fiercer, and more confident than ever before. And it feels great - right up to the moment when he loses control of his own strength and almost kills another player during the soccer team tryouts. An incident which proves to be just the beginning.
Jack is about to learn the startling truth about himself: he is Weirlind; part of an underground society of magical people who live among us. At the head of this magical society sit the feuding houses of the Red and the White Rose, whose power is determined by playing The Game - a magical tournament in which each house sponsors a warrior to fight to the death. The winning house rules the Weir. As if his bizarre magical heritage isn't enough, Jack discovers that he's not just another member of Weirlind: he's one of the last of the warriors, and his power has manifested at a time when both houses are scouting for a player. Jack's performance on the soccer field has alerted the entire magical community to the fact that he's in Trinity. And until one of the houses is declared Jack's official sponsor, they'll stop at nothing to get Jack to fight for them ...