Wasn't the Twilight Thorn just a computer progam made up by DiZ? To help Roxas remember he was Sora. He was probably a nobody because
1. Roxas was a nobody,or 2. DiZ wanted to test his skill for destorying the nobdies.
Then again, DiZ might have been too afraid to program heartless into the world. Everyone that he programed into the machine would have been effected right?