Xander Cage, aka xXx, is an agent of the NSA.
Unofficially, of course. Definitely unofficially, so much so that no one at the No-Such-Agency would ever admit to knowing him let alone associating with the likes of Cage, outlaw and renegade. And Cage would hand that feeling right back at any suit who had the nerve to face him. Cage did not like suits. He did not like authority. He most definitely did not like anything related to law and order.
Cage "lived for adrenaline highs", where that chemical made him "feel alive like nothing else in the world ever did". He got that need for the rush as a young kid in a family that barely scraped by so there was little to make his life easy. This meager lifestyle would easily explain why he learned to make his own entertainment but not his obsession with danger and excitement. That came from inside him and at the age of nearly 30, it had only intensified.
So with the constant need to push it to the edge and often, literally, over it, Cage could have picked several extreme venues. Crime was definitely one he might have chosen but did not. Breaking the law, on the other hand, went part and parcel with the avenue he did choose. Technically that did make him a criminal but not in the traditional sense.
Cage went for outrageous stunts, activities that, should he make the slightest miscalculation or be faced with an x-factor from out of nowhere, it would be all over for him. Then he had them videoed. And then sold. And his reputation as a crazy man doing crazy things for other would-be crazy people to watch grew.
Which is what brought him to the attention of Augustus Gibbons, a special agent with the NSA and a man tasked with pulling off some pretty insane missions. Saddened by and tired of losing highly trained agents in the field, he decided that some of his assignments did not really call for men trained in spycraft and skilled in cloak-and-dagger work. These suicide ops needed people who might be ruled suicidal by normal people but who really didn't want to die - they just wanted to be extreme.
People like Cage, so in love with eXtreme he tattooed the letter three times on his neck.