Cripple Wa was the owner of a floating crap game, at which the most desperate gamblers in Ankh-Morpork habitually lost vast sums. He was once foolish enough to annoy Death who allowed Wa to see Him without the neural blinkers humans always use. He needed more than just to change his underwear...
The first appearance of Cripple Wa is in fact in the first Discworld book, The Colour of Magic, in which he is one of the two beggars keeping watch on Pearl Dock. When the sharp eyes of Blind Hugh detect a stranger getting off a ship, Cripple Wa hurries off on his fast, nimble legs to carry the news. It is to be assumed that both beggars are what, in Victor Hugo's Hunchback of Notre Dame would be called "Bogus Cranks" (i.e. pretenders to disability for begging purposes).
In the 2008 film version of The Colour of Magic, Cripple Wa was played by cult actor Allin Kempthorne, best known as co-creator of the film 'The Vampires of Bloody Island'.