The voice inside my head

Luke's older sister, Pat, has always been his moral compass, like a voice inside his head, every time he has a decision to make. So when Pat disappears on a tiny island off the coast of Honduras and the authorities claim she's drowned-- despite the fact that they can't produce a body-- Luke heads to Honduras to find her because he knows something the authorities don't. From the moment of her disappearance, Pat's voice has become real, guiding him to Utila, where she had accepted a summer internship to study whale sharks. There Luke meets several characters who describe his sister as a very different girl from the one he knows.