The only fleshed-out background of Willie's character was in S.E. Hinton's DS novel, Hawke's Harbor. It was to be a part of the Collins-Harper rebooted series of DS novels, but after just a few the project collapsed. From what I read, the book was to be called Dark Shadows: Collinsport. It covers Willie's background and what led him to where he ended up. The climax is heartbreaking. I actually cried.
Gerard