Double Two-One is a Special Agent with the Special Investigating Branch.
The SIB is an interplanetary Intelligence and Police bureau one hundred years in the future (from the time the recorded adventures were published). The head of that agency is Air General Zodiac, previously named Steve Kalinski and now referred to simply as 'S'. That gentleman is the one who decides which agent will be assigned to what missions and in his mind there is no doubt that the best of his many agents is Double Two-One.
If this agent has a real name, we are never given it and in the very short two missions we follow him on, he certainly does not need it. There is nothing personal ever mentioned about this intrepid operative outside the fact that he really comes across as strictly business and not one for small talk. This is especially true in the second assignment when he is pulled suddenly from his vacation to look into the problem and he is not happy about it.
Keeping in mind the severe lack of info about this agent, we are told he has "cold blue eyes'. When given the details of his assignments, he maintains a solid emotionless demeanor and even when being told how dangerous the task is, he doesn't blink. Either he is that confident, that cocky, or that clueless that he does not have to make any comments.
One person who is forced to work with Double Two-One and does not like it regards the agent as a "squared-jawed, public-school officer worker who thought he was such a great Special Agent".
Spoiler Alert: Shortly after an unfortunate result of the second mission, Double Two-One's field career is ended in an accident but his role in the organization will be elevated to be the position of Chief Officer for all Special Agents. From that point on, it will be he who calls the shots in assignments and he who will be the one to receive any queries from them when they "Contact Twenty-One". His designation will be Twenty-One (which will be strange because there will also be a in the organization.