D-13 is an agent with American Intelligence.
His real name is Richard Anthony. He is a Captain in the military. His relatively young appearance at the start of the series would lend one to think he was in the Army (Air Force did not yet exist as a separate unit and his hair cut would never have been allowed in the Marines). He is frequently seen in an Army uniform so it is probable that he is still on active service but he is more often seen in civilian clothing which would indicate he is attached to an unnamed government agency. He regularly reports, according to him, to "Intelligence Headquarters". Later in the series, though, there is several references in different cases to his being of "Naval Intelligence" which would greatly change his rank level (O-3 to O-6). Later still mention is made of his being part of the Secret Service. So, exactly who he works for is really up in the air.
His codename must be known as he uses it to summon help by radio in his third adventure and in the fourth, a hijack leader calls him by that upon first meeting. Some time later when he encounters agents for yet another country, one of them remarks that "it is our old friend, Captain Anthony" indicating he is by no means a newcomer to the counter-espionage game. On another instance he is addressed by name without having divulged it, showing he is known to numerous enemy agencies.
We know virtually nothing about the good Captain. He is likely in his late 20s based on his appearance. He has wavy blond hair and is clean shaven. He is very athletic and agile and certainly knows how to handle himself in a fight. Jujitsu is probably one of his skill sets which he uses a lot and he is a crack shot. He appears to act without fear when the situation calls for it but to his credit he does not rashly charge into a fray when stealth might be better warranted.
One other quality that helps define Anthony is how the ladies take to him. More than one member of the fairer sex is drawn to him and it does not matter whether the woman in question is a "good-guy" or villain. His quick smile in reply to their overtures clearly shows he is by no means adverse to their attention.
One interesting aspect is that two of the ladies that have the most influence are both Canadian Intelligence operatives. At first there is Helen Cameron who is in several adventures (issues 11,14,16) and then she was gone. Very soon thereafter, the equally intrepid and gorgeous Lorraine McAllin makes her appearance (issues 18,19,20,22,23,24,25,26). She will stay around until the end of the run, eventually ending up the fiancée of Richard Anthony and, in the last adventure, an American agent alongside him.
An oddity that is never explained occurs in the 15th adventure when at the end, having saved the country "from an insidious menace" the President appoints D-13 "chief of the Pacific Intelligence". The next month's adventure has him back at work as normal.
ComicVine, the great online source for comic book information, says it best with its one-line description of the series: "His heart was true, his fists were ready, and his teeth were strong and white, other than that there is not much you can say about D-13."