The Runners of Abra by Shaun Stevenson

When Eric was only thirteen, he was taken from his family and the peaceful rice fields on the island of Jedros to become a Runner. Roaming the five islands of Abra, Runners are tasked with jobs -- jobs they must see to the end. Either finish the job or die.

And then a mysterious benefactor arrives with a bag of gold and Eric's first job: find the girl spotted somewhere in the northern islands responding only to the name Bella. Simple enough. But this job is not what anyone thought. Others are searching for the girl. Others who will kill to keep Bella a secret.

​But Bella has her own secret to keep. And if it gets out, the very fabric of the known world will change forever.