Betrothed Vol 1 - Sean Lewis - Bog

Kieron and Tamara are both seniors at the same high school. They've never paid much attention to each other, even though they are the only kids at school who are orphans. On the eve of their eighteenth birthdays, they discover a lifelong secret: in a dimension far away, they are each the leader of an army that is at war with one another... and what's more, when they turn eighteen, they're to be Betrothed or Fight to the Death!