Done It Duncan
Duncan Did It. He Does every major crime in Ankh-Morpork. According to him, anyway. A totally useless thief with not even the brains of a pigeon. Even tries it on with a Seamstress idly swinging a handbag in the Shades. This turns out to be Nobby Nobbs as a decoy while the more morphologically gifted Sergeant Angua lurks in an alley.
Carrot calls him the worst thief in Ankh-Morpork. As in, least good. Not most prolific, no matter what Duncan's protestations are. Often Does It so he can have a spell in the cells, where at least he's fed.
Only the second creature ever to have had Nobby feel sorry for, after the goat in Feet of Clay. He gives Duncan some small coins in order to feed himself. Angua can't believe it either, but Nobby backs up his image with a "Werl, I can always get it back from the Petty Cash..."
It seems that, in his eagerness to Do every major crime, Duncan has his ear to the rather greasy ground of the city's criminal underworld. This at least gets him a passing mention as an informant during Jingo.
Nobby remembers him well enough to mention him in passing in Raising Steam (p. 284).