Rollercoasters Noble Conflict by Blackman
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 is 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 named Rhea. Haunted from that moment on by strange visions and memories, he realizes he hasn't been told the truth about what he's really fighting for.