Desmond Okewood is an agent with the British Secret Service.
Part-time, perhaps. Or possibly he can be considered on unofficial loan to that Department from his duties with the Royal Army where he holds the rank of Captain. As he puts it rather angrily some time after his 'conversion', "I may have been a Secret Service agent by accident but I'm a soldier by trade. My place is in the fighting-line!"
He is not on the front lines when we first meet up with Okewood as he is occasionally suffering from what he refers to as shell-shock. "You think you are cured, you feel fit and well, and then suddenly the machinery of your mind checks and halts and creaks." It is more likely that he ails from a brain injury considering the fact that he only recently "left hospital convalescent after being wounded on the Somme ('gunshot wound in head and cerebral concussion' the doctors called it)".
The year of our initial time with Okewood is likely late 1916 and Okewood is on medical leave when he learns enough to be worries about his brother, Francis. Interestingly, which Desmond Okewood would on more than one occasion grouse about being with Intelligence instead of fighting, his brother is and has been since the beginning of the War, an actual operative. Francis had failed his induction physical and was left with only one way in is mind to do his part in war effort and that was to join the Secret Service.
It is the search for his missing brother that will get Desmond involved in Intelligence work and as time passes, he will remain with that line of work because he has shown that like his brother, he is good at it.
It is also in that search that Okewood would come upon the man who would be considered for quite a long time his nemesis or archenemy, the evil and quite formidable Dr. Adolf Grundt, also called Clubfoot due to his physical infirmity. While there will be a mission or two that Okewood will be a part of that does not pit him against Clubfoot (that is the normal way he refers to the man), it is without a doubt Dr. Grundt that will cause him more trouble than everything else.
One final comment on why Okewood would continue to work in Intelligence even after he could have returned to active duty. A woman that Okewood respects comments to him, "The Secret Service has its fighting-line, too, though the war correspondents don't write about it. It never gets a mention in dispatches, and Victoria Crosses don't come its way. The newspapers don't publish its casualty list, though you and I know that it's a long one. A man slips quietly away and never comes back, and after a certain lapse of time we just mark him off the books and there's an end of it. But it's a great service; and you've made your mark in it. The Chief wants men like you. You'll have to stay!" He agrees.
Good line from The Fox Prowls, an aged Okewood (not exactly sure which one), now Chief of the British Secret Service, admonishes a young agent, "Don't get married as long as you're in this job. No fellow in this game ought to marry. The wives never understand."