Damien seems innocent at first. He's a goofball archeologist who finds Katy Devlin's body in the woods, and he's stupid enough to touch it, contaminating the crime scene. Rob doesn't understand how a dork like Damien could function in society, much less be a murderer, so the big D (for Damien, not dork) flies under the radar for most of the book. But, thanks to Cassie's interrogation skills, Damien ultimately confesses to killing Katy. Oops.
The scary thing is how much he doesn't seem to care. He murdered a twelve-year-old girl, yet he still maintains his carefree attitude, as though this murder was just a blip on the radar. It's a bloody big blip, dude. Plus, he's a little more cunning than Rob ever gives him credit for. That whole touching-the-body thing wasn't goofy ignorance; that was carefully calculated so that Damien would have an excuse if any hairs or cells were found on Katy's body. In other words, Damien is totally the guy whose neighbors would say, "He seemed like such a nice kid." Thing is, it's the nice kids you always have to watch out for.