My favorite NPC is Mulligan, the owner of Mulligan’s Pawnshop in a homebrew campaign I ran, although he could really be put in any town or city. Mulligan speaks with a cockney accent, and only has one arm (Lost ‘at one to a shark ’bout 10 year ago, mate, hafta shake me other ‘and I’m afraid!). After talking to Mulligan for more than a minute, you definitely get the impression that he is a bit touched in the head. He has converted his home into a pawn shop and now sleeps in his attic (which he refers to as his loft). This is a good place to go if you have some bandit armor and weapons to unload, or are looking for some unusual piece of equipment. Mulligan even claims to have a few magic items for sale, showing a long sword with continual light cast on it to anyone who will have a look (the sword is obviously not of a quality to be truly magical). In general, Mulligan fills the niche of village idiot while also providing a useful service to the community.
The best thing about Mulligan, of course, is that all of the above is just an act. Mulligan is really an accomplished fence, and a retired master burglar. Only people who truly gain his confidence ever get to see behind the facade that Mulligan has worked hard to create. Not only has Mulligan turned his house in to a shop filled mostly with junk, he’s turned his basement into a storehouse filled with all manner of valuables, from priceless art and gems to perhaps a few real magic items. There is also secret access to the sewers from here, and Mulligan can make it to and from a dozen thieves guild safe houses without it ever looking like he left his shop.
Mulligan moved to the city he currently resides in several years ago and has worked hard to maintain his village idiot persona. Much of his past is a mystery even to his closest associates, but it is likely that all of his stories about being a merchant mariner and loosing his arm to a shark are indeed just stories. No one would believe a person who tried to out Mulligan, who has acted like a lunatic for years and years; anyone claiming he was anything other than a lunatic would look to be crazier than Mulligan acts.