Nearing the end of Darkly Dreaming Dexter, Dexter has discovered that his mother was brutally murdered in front of him when he was very young and his adoptive father found him at a crime scene.  Another important detail Harry did not tell him was that Dexter had an older brother, he was found at the crime scene with Dexter, he was put in an institution and never heard from again.   The Ice Truck Killer is still on the loose, but what is worse is that he is still making some form of contact with Dexter by breaking into his house and leaving things, as well as going through photos from his childhood and recreating crime scenes. Dexter seems to have become fond of him, saying “That’s why I liked him, I think. Another guy pretending to be human, just like me” (Lindsay.)  Dexter has a real attatchment to this guy, as he seems to be the only person around who knows what, or who, Dexter is, and is not scared or out to get him.   My guess is that the Ice Truck Killer is Dexters estranged brother, and he is trying to meet him not harm him.