Slade Heller is an agent for the Elysium Group.
First, about that organization, it is important to point out that it is a private intelligence organization; a for-profit corporation that "was registered as a Private Military Company but functioned more like a private intelligence and tactical operations firm. The company worked for the highest bidder, but did its best to be on the fairly good side of events, despite hiring the worst form of scum for their workforce." It is headed by Troy Smith, a dark-skinned bald old man who is also the man to have seen something useful in Heller and brought him into the firm.
Second, about Heller, it is equally desirable to be honest that Slade Heller is not this man's real name; it is the name he chose to adopt when he chose to leave behind his old existence and start working for Elysium. "Still in his early twenties, he guarded anything connected to his past and his birth name with ruthless ferocity." Even more telling about the man is that he knew "he'd do anything to protect this identity".
When he says "anything" it is wise to take the man at his word because Heller does not spy for Elysium, he kills for them. Heller is a trained, and by the time we meet him, already fairly experienced assassin. He kills people. He does it quickly and efficiently and he does it coldly. He also is very determined to not be bored so when he is once told about a particularly dangerous mission that he was free to refuse, he accepts the job saying, "I'm game for anything that beats attending a training module and doesn't send me to prison. No questions asked".
As the chronicler of these adventures puts it: "Slade Heller is a devilish, anti-heroic assassin who's not evil but he's definitely not a hero either. His dark and demented soul tries to do good and kill evil people for the right price but he has no delusions of morality. He brings out a lot of dark humor, but also has a twisted shred of a moral compass in him. He is different from the usual action heroes of the genre who have a background in the military or the intelligence community as he has none of it. He's an enigma and will remain that way. Slade's also younger and shorter than most of the action heroes we usually read, and that was a fun experience for me to write."