X-5 is an agent with American Intelligence.
Which agency he works for is never mentioned but in the first adventure he is clearly identified as a U.S. secret agent. In the subsequent escapades his employer is not really specified. That he is an established agent is revealed in the first adventure when an enemy agent, certain that the man he was observing was a spy, looked into his book of known agents and pulled X-5's picture.
Who X-5 is outside of his codename is never mentioned. No one ever uses a first or last name and he seems very comfortable with that. What his life is like outside his work is also never even alluded to much less stated. The fact that he seems to be all over the place would make one believe he does not have much in the way of a personal life.
We can infer from some of his adventures that he is a gifted disguise artist and he has to be an expert with foreign languages to be able to converse so easily and, while made up to look like an enemy general talking to his fellow countrymen, so convincingly. The fact that he makes decisions to go undercover so quickly and with little trepidation would point to his having done it many times before.
His experience and his reputation must be extensive for he is obviously well known. When a European nation of Ruina is in trouble with an aggressive neighbor, the King demands "Cable X-5! I need his help immediately." When word gets out of his coming, the bad guys know they have to be ready for him (of course, they also know when he is arriving and how). He is able on another mission to
Physically all we know about X-5 is that he is blond with wavy hair, prone to wearing suits, and in good condition based on his ability to dodge bullets (most of them) easily. He is darned good in a fist fight and really acts like he relishes a good tussle. He is also fairly glib for he has a sharp comment or retort always at the ready. He is definitely not above making a biting remark when it is called for.
In the first two recorded adventures, he is called "X-5, Secret Agent". In the last two, he is "X-5, Super Agent". There is no indication what changed to make him move from secret to super.