Not that you should try it here, but something I've always wanted to pull on my players... a kid.
Just a normal, no-ability, teenager with a really high CHA (Expert 3 or so, with most of the skill points going to Bluff and Knowledge (local)). Lousy equipment, physically weak, that sort of thing. Nothing that'd cause a powerful creature to see any sort of threat.
How many people put ranks into Sense Motive, anyway? Besides Palladio, that is.
They're in this place in the Underdark, full of horrendous creatures that gives these divinely-empowered paragons of light the willies, and in the middle of it all is someone who's managed to convince everyone that he's really powerful simply by the obvious fact that if he wasn't, he wouldn't be there. (see also: Somebody Else's Problem field, Invisible Pink Unicorns)
After all, anyone that obviously weak has either massive innate abilities or very powerful friends. So, everyone has simply assumed that this kid is not someone to be trifled with.
Give him an item that prevents scrying and mindreading, and he bluffs his way through it all (maybe an item that gives +10 or +20 to Bluff). He's smart enough to stay around intelligent races (since you can't bluff an animal to not eat you as easily), and is mostly just seeing the sights.
Originally this basic concept came from an old campaign, except there, he also had a curse that gave him immortality in the Wile E. Coyote fashion; that is, no damage was ever fatal. Add this to the "When Wishes Go Bad" file under "eternal life".
Set off a trap, he'd be fine in a few minutes; he wouldn't gain XP from any encounter in which he died, so he pretty much always stayed a low-level NPC. But, he had all the accumulated memories of a lifetime of reckless adventuring, so he could easily come up with plausible lies.
I don't think this ability is absolutely necessary to the concept, but YMMV.
