Stacy Martinez is an agent with the Citadel.
Well, sort of. As often is in life, it's complicated.
She will be working mostly with them for most of the time we spend with her but when we first meet her, she is an agent with the FBI and she was pretty well dedicated to stopping any agent of the Citadel that she could find. Not that she could find many because they are especially well trained to stay out of the limelight. Life, though, will cause her to say goodbye to the Bureau and join up with the Citadel.
We meet her initially as an FBI agent on the hunt for a Citadal operative named and it is in the write-up for that man's adventures that we say of the Citadel 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".
Half of the first Greene adventure with Martinez as a major player, she and her long-time FBI partner, Bill Logan, first seem to be assisting Greene (well, at least getting him released from the custody of a county sheriff) but were actually after him and likely others in the Citadel. Over time, her career path will take a major shift and she will take a bit of a different attitude towards the Citadel. "She quit the FBI because she got too close to the truth, and they wanted her to stop" was how one fellow she meets puts it to his buddy. He was kinda close. For more information, you need to follow first Greene and then Martinez.
Now back to the sort of working with them. Her relationship with the Citadel is an interesting one. It has an ongoing conflict with a group called Axion and she does, too. Which is why she sides with the Citadel and why she accepts their help when she needs it. But she has also sort of helped herself a wee bit in the first adventure she has on her own. As we follow her actions, they will take us on an interesting side trip. The Citadel maintains a series of PO boxes around the country used to fund its agents as needed. "During the confusion caused by their war with Axion, I had raided several of the PO Boxes. As far as I knew, no one had discovered the missing money, or if they had, they probably assumed it was Axion". See what I mean about the relationship being interesting?