Kyle Butler was an alias which Dexter used, when he introduced himself to Arthur Mitchell. Kyle Butler is also the name of two minor characters who only appear while Arthur is searching for Dexter, in episode 11 "Hello, Dexter Morgan".
Season 4
Dexter used this name in order to get close to Arthur Mitchell. After Dexter stopped Arthur from killing Scott, Arthur tried to find him. However since Kyle Butler wasn't Dexter's real name Arthur stumbled upon the real Kyle Butler, while trying to break into his home. Kyle threatened to call the police, which left Arthur no choice but to kill him in his home by bashing his head. Later Dexter with police arrived at the crime scene. Thinking that Arthur will continue to look for "Kyle Butler" Dexter subdues the next Kyle Butler on the phone book list so that he can get Arthur, when he comes searching for him. However when Trinity breaks into Kyle's home, he sees Kyle's picture on a Christmas card and leaves, knowing that he is not the right person.