Xavier Greene is an agent with the Citadel.
One brief passage concerning this group stated that "it was a mysterious organization with seemingly no boundaries or oversight", which definitely sounds ominous. We also learn elsewhere that it was "a power conglomeration of influential people, corporations and even countries that operated in the shadows. No one knew exactly what the Citadel did, but their influence spanned the globe and rarely was a major policy decision made anywhere without their input".
As one member of the organization puts it, "The Citadel is set up as a bunch of Stations. Each Station is a completely independent hub. They each have their own safe houses, garages, resources and are completely separate from each other. They all communicate with each other, but they aren't connected. One Station normally has no idea what the other Stations are doing. They all operate completely independently. Only the heads of each Station get together and coordinate. That way even if one is infiltrated, it won't endanger the others.
Greene has been with the Citadel for a lot of years, recruited by it when he was quite a bit younger and hired to serve "primarily as an assassin, taking out assigned targets without questioning". As time passed, his ability to handle the considerable pressure of his work as well as his impressive skill at problem-solving honed his talent so that "he became known in the intelligence community as 'the Silencer', a highly specialized agent who had the ability to seemingly appear and disappear without a trace. Eventually, he was only sent on the most important and high stakes missions, which afforded him better paydays and less of a work schedule".
Then came the mission in the Himalayas which would alter his life considerably. During a chase through those mountains after having retrieved a hard drive, Greene was caught in an avalanche. The Citadel thought him dead but he had been rescued by a monk, Quan-Li, and nursed back to life in a secluded monastery. During that long recovery he trained and studied with the monk and when after a considerable amount of time he decided to make his survival known to the Citadel, it was as a changed man.
He still does the occasional job for them but when he is finished, he returns to those mountains once more.
Playing a major role in each of the two Xavier Greene adventures before getting her own starring role with Greene showing up in a cameo capacity is determined and stubborn, albeit highly resourceful FBI agent .