Justin March is an agent with British Intelligence.
Above in the fact section, the department for which he works is labeled as "Y.2" though the breakdown of that designation is not given. The man who is the Chief of it is also described as being the "Intelligence Chief" and his organization as "the Department of British Intelligence". Normally I would have listed that there but "Y.2", as enigmatic as it is, looks cool.
Whatever the true designation is, March is very much an experienced and highly prized operative in the group, so much so that he has earned the nickname of "the Ace" which is said to be used not only by his fellow employees of the Intelligence communities but, according to the narration, by many others "throughout most of Europe and America". As such we might conclude that while he is a superb agent, he is not a "secret agent".
I have listed March's nationality as British but it is important to note that he says he came to England "from his native South Africa" where he had served with distinction under General Smuts (a prestigious officer and statesman who served two terms as Prime Minister there).
March is described in at least one spot in rather glowing terms as having "that indefinable and unmistakable stamp which can only be described as 'quality'; [he] might have stepped, apart from the clothes [he] war, out of a bygone age - an age in which chivalry went hand in hand with adventure, and men fought at the first glimpse of a side-long glance, or of a disparaging smite at the mention of a woman's name".
While I found no actual examples of the last parts, there are more than one occasions when March is found standing watch over a situation, practically pleading for a reason to step in and manhandle someone or, better yet, make use of his prowess with his sword cane!
- When cautioned by his boss that the woman he was going up against was as dangerous as she was beautiful, March responds, "I take it, sir, is that she's a beautiful bitch? I don't want to boast, sir, but I make a specialty of beautiful bitches".