Alistair Granby is an agent with the British Secret Service.
When we first encounter him, he is definitely an operative for that intelligence organization, and a mighty good one at that, but as time progresses he will assume far more of leadership role and take on the duties of sending others out into the dark alleyways and back rooms to gleam the hidden information the government needs to best govern, making him fit the spymaster role better. Since, however, most of the adventures we have of him put him solidly and dangerously in the field, we stick with the agent label.
"Steady, the Buffs!" That is the second sentence we will hear Granby utter, the first simply asking a young man what was going on, in The Six Proud Walkers. The exclamation was a curious one to me so I looked it up and found it was a reference to a British regiment dating back to the 1500s and the phrase has come to mean simply "take it easy!" or "relax". Granby would enjoy saying this on more than one occasion when tempers flared and people got agitated.
"Pretty Sinister" is another turn that he will use quite easily in the same conversation, this time referring to a situation that is interesting and possibly dangerous and, for Granby, well worth looking more closely into. Seldom would we ever find Granby being foolhardy but he is perpetually curious and prone to looking into things others would have avoided.
The gentleman who narrates the first adventure we read of Granby, a British fellow named Geoffrey Carroll, had just had a series of unpleasant and potentially deadly incidents befall him and when he came across one of those causing the trouble and tried to talk with him, that man pulled a knife. Carroll would have died then had not Granby thrown a solid punch of Carroll's shoulder and onto the miscreant's jaw, putting the latter on the ground dazed.
It is in the aftermath of that encounter, when both Granby and Carroll are safely away and able to relax, that we learn Carroll's impression of Granby. He describes Granby as having "a pair of the most piercing blue eyes I have ever seen, blazing out of a thin, tanned face". He goes on to note that there was a "twinkle in those eyes". We learn that the rest of Granby is as thin as his face and that he is of relative short stature, likely around 5'6" because when Carroll commented about another individual being that height and being short, Granby mildly bristled that it was hardly short since it was also his height.
"Haven't we already shared God's best gift together, which is a hearty scrap", Granby tells Carroll and in those words we get an excellent view of the mindset of Granby, a love of adventure and daring and not a little mayhem.
At another point when Carroll was stunned that Granby believed his "improbable" story, Granby quipped, "I don't expect things to be probable. Nobody does who knows anything about them."
Granby, trying to get a description of a man, asked Carroll if he had observed the man's ears and when told he had not, directed, "Always remember people by their ears when you can see them. There's nothing so distinctive." Granby would also lament the normal inability to really notice things with "Nobody ever sees anything unless they have their noses rubbed hard into it. It's painful, but it does'em a world of good."
We learn early on that during WWI, Granby had operated deep in enemy territory in what is now Albania and further north in the Balkans, working to undermine the other side as best he could. He had done well and made a name for himself, shown by the fact that when the adventure required meeting with the Prime Minster of Italy, that man commented he had heard of Granby and his successes.
Since the War, though, what Granby has been doing really depends on who he is talking to and all likely not true as Granby likes to make himself sound like an opportunistic entrepreneur instead of the intelligence man he really is. In The Five Flamboys, Granby describes his organization as the "Secret Service. Most amphibious. One foot in the Foreign Office and the other in Scotland Yard. And always very private and confidential." With others, though, he acts more like an outsider who just likes to meddle.
The truth is, as we learn going along, Granby is a very important part of British Intelligence and his importance will grow as the series progresses, becoming the head of his very secret department and, for the most part, running the Intelligence system.