Doc is, as his name suggests, a doctor. In fact, he's presumably one of the few doctors in Western Mudos. Being a Vykker, a creature of such a shady descent, not many Clakkerz seem to trust him, and thus, he doesn't get many patients. His prices are ridiculously high (perhaps because there aren't a lot of people willing to come in his office to do business).
Doc seems to be the Stranger's only hope if he wants to have a mysterious procedure performed to save his life, and also seems to be the only one who truly knows Stranger's dark secret. Doc presents him an offer for a 20,000 Moolah operation at Gizzard Gulch. Stranger goes out to save up for the operation. While he is in the Mongo Valley, he is informed that Doc wants to see him. Stranger then heads to Doc's office to find him hung by the outlaws. It is rumored that Doc once worked for Vykker's Labs.