Personally, I have some kind of military background on him, just because of his "expertise" with weapons. When I play VC again, I may also have him somehow related to Tommy Vercetti, if only because they look quite a bit alike. As for the name... John Doe works oh-so-well.

Also, your own theories as to why he doesn't talk? I've seen some speculation that the reason is due to his getting shot in the opening credits (Eight Ball has the hands, The Guy has the vocal damage). I personally think that works fairly well. Your own thoughts?