Malorie Blackman 
Noble Conflict 

Years after a violent war destroyed much of the world, Kaspar has grown up in a society based on peace and harmony. But beyond the city walls, a vicious band of rebels are plotting to tear this peace apart. It is up to the Guardians — an elite peacekeeping force — to protect the city, without ever resorting to the brutal methods of their enemy.
When Kaspar joins the Guardians, he has a chance encounter with a rebel — a beautiful girl named Rhea. Haunted from that moment on by strange visions and memories — memories that could only belong to Rhea — he realises he hasn»t been told the truth about what the rebels really want, and what he»s really fighting for.