James Winchester is an agent for Blackwind.
Blackwind is a British private security company which "only dealt with the major offensive operations, typically on behalf of governments and other shadowy figures". Need a particularly nasty mission handled and not want it to come back on you in the press? Contact Blackwind which would likely in turn contact someone like Winchester.
Winchester enjoys what he does for Blackwind, just as he enjoys the "ungodly amounts of money" he earns working for them. He receives that large income because he eliminated targets, that is to say he killed bad people, on behalf of Blackwind. Since these targets obviously would not want to be removed, Winchester had to work hard to earn the large sums he did. While he enjoyed earning what he did, he is honest enough with himself to know that he did the sort of work he did because "his career choice had prepared him for a life in dealing death".
We first meet Winchester as he is living in Guanajuato, Mexico, a place he chose because it was a long, long way from England and Blackwind. He was not hiding, per se, just in need of a rest after a couple of particularly difficult missions. Then again, every mission he got from Blackwind was particularly difficult. It is interesting that the next mission brought to him by Sinclair Wood, his best friend and handler at Blackwind, also happened to be in the same region.
We do not learn a great deal about Winchester's history or how he came to work for Blackwind and its founder and CEO, Gallagher. We get a glimpse of an interesting past, though, when we are told that he started work for Gallagher when that man got him out of a military prison and offered him a job. Since then, though, their relationship seems to be a strangely edgy one with Gallagher routinely giving him difficult missions that look certain to getting Winchester killed and Winchester stubbornly refusing to fail.
Winchester is described as having bright forest green eyes and a "handsome countenance and hollowed cheeks [which] told the story of a high-stress lifestyle. His shirt traced the outlines of rippling muscles held to a sturdy yet slim frame". He is basically a loner. Said of Winchester by Sinclair "I know you're the type who doesn't like humans." Winchester did not argue.