If you're talking about the guy in the alcove in the east side of the base who doesn't talk you, I heard from somebody that he was supposed to be a quest that was taken out, he won't say anything to you, and if you leave, and come back, he's dead...? Could be wrong tho.
As I remember it, the quest was after getting the ball gag from Francis, to use it on Shadow-Who-Walks, and make him scream. Then the raiders would let you join them. Or something like that.
Doesn't anyone read the phantom? It's an easter egg: Shadow-who-walk, Ghost-who-walks? Do you hear a bell ringin? He's probably part of some removed quest.
"If pigs could fly, the teachers lounge would be the JFK airport"
-Kilroy was here-
naw, the guy who said the shit about him being a guide is right. If you talk to (either bishop I think, or maybe Sargeant stark in VC) he'll tell you about how the raiders must have a guide that takes them through the canyons and stuff to a hideout. Shadow Who Walks is the tribesman who does that, and that ball gag thing doesn't work. Whitechocobo666 put a mention of it in his Evil Walkthrough but he says that it didn't do anything when he snuck up and used it.