Agent V is an agent with American Intelligence.
More specifically, it is likely the agency for whom he works is the United Intelligence Division. I say likely because we never actually know if he works for that department or is in fact a freelance agent.
In the first recorded adventure, he is asked to perform a service for French Intelligence. In the second tale, it is for British Intelligence he works his magic, although he is apparently being on loan. The third story is where we learn of the UID and his job for them but again, it is not actually stated he was their agent, just that he was performing a mission for them.
Mystery seems to be something this operative enjoys. We surmise that he is an American because he is identified as such by several people he deals with and each of his missions has him taking a false identity as a U.S. citizen but he could be of any nationality. For simplicity sake, we will stick with American.
Obviously since we are referring to him only as Agent V, we have no idea what his real name is. Neither, it would seem, do the people hiring him for they just call him by that letter.
We know nothing to speak of about him except for a short description repeated in each of these tales, talking about how it was hard to discern his age due to his "white and lineless" skin, each commentator putting him somewhere between early 30s and his 40s. Also mentioned was that his ability to portray himself as a man used to ease and comfort, belying the fact that hidden was his "superbly trained athletic body that could spring into action in the fraction of a second".
Also known about V is his bag of gadgets. Two are mentioned though more are likely to exist. The two used were small gas bulbs which, when smashed on the floor, explode and give off a greenish gas causing discomfort to those near it and a special mirror set up on his wristwatch allowing him to watch his own back.
And then there is his catchphrase, used when he has succeeded in foiling a communist plot somewhere in the world, "V for Victory, the Victory of Democracy over Red Tyranny."