John Purkiss is an agent with British Intelligence.
He is more accurately a "Ratcatcher", a term used by his boss, Vale, and himself to describe someone who goes after the rats in the house. Most of the agents in MI6 are loyal subjects out to keep their country as safe as they can but some bad apples sneak into the barrel and some turn bad over time. When they are discovered, it is the job of Vale to ferret them out and the job of John Purkiss to hunt them down. Often eliminating them is the only alternative and Purkiss is not one to become squeamish.
He was not always quite so ready to partake in violence. When he was younger, he joined MI6 as a standard agent and expected to remain as one for a good time to come. He met and fell in love with another agent, Claire, while stationed in Marseille and they became engaged. Though interoffice relationships were frowned upon, Purkiss managed because after five years of being an agent, he was good at hiding.
Then Claire told him of her suspicions about her boss, a senior operative that Purkiss happened to like and respect. He did not want to believe but Claire was insistent and as they started to watch him, proof started to stack up. Then things went bad quickly and Claire was killed. The death of his love by a fellow agent changed him tremendously. He was furious that someone he had trusted had taken something so precious and he was thoroughly disgusted at the department that let the man operate. He quit.
It was shortly after that the aged and experience Vale came to visit. He was offering a job. Just as the man who had killed Claire had needed catching, so did others and when your career is spent in the shadows, being hunted down is not easy. It requires someone very good and very dedicated to take on that task. Purkiss still had a fair amount of anger left but he also had an emptiness that cried to be filled. Doing something was far better than just sitting and thinking what might have been. He took the job and stepped back into the shadowy world, this time as a hunter instead of a watcher.