He may be G-Man, he may be a trained weapons and tactics professional without any kind of real scientific education, he may even secretly be a morphed headcrab-turd from outer space, but he's definitely not Ali-G. Closed.
I read the secret 3000-page Half-Life Bible, he's Hordon Freeman's long lost twin. They fight together in Half-Life 3, but in the end they are forced by their father, G-Man, to battle eachother to the death. Whoever wins depends on which character you choose to play as, and there's a co-op mode. *
*: False. You actually play as Adrian Shephard in Half-Life 3. Let the flamewar discussions commence!