Doctor Kallahan was the twin brother of Killer Kallahan, but started out with good intentions. He had been doing pretty good at medical school, but had to drop out due to lack of funds. Where his brother went bad, the Doc had intended to go good. but in this town, nobody stays good for long.
He was known as 'The Doctor' despite the fact that he never obtained his license. That didn't matter here, because there was plenty of work for someone with his connections.
He had established a network of drug smuggling, and was the man to see if you needed anything. He was also the man to see if you wanted someone poisoned.
He knew that working both sides of town would get him the most work, so he never took sides. However, the sides each wanted to take him, though he couldn't be bought. His bedside manner kept him straight in the middle, which was exactly where he wanted to be.