Danny Nash is an agent of MI6.
Operating in the years before the start of the Second World War, Nash is one of those operatives for whom the term 'gray man' is used, likely because while he works for the Firm, he also doesn't. "Sooner or later his luck would run out - and when it did, the powers that be back home wouldn't be mounting any rescue. He was on his own".
Nash is technically an officer in the British Army but since at the time of these adventures, roughly 1933-34, MI6 really is Military Intelligence Department 6, his being assigned to that group is not surprising and was without difficulty. Nash holds the rank of Major in the service which is impressive because from the way he is portrayed, he is still in his 20s, albeit towards the end of that decade.
In MI6 working directly for the head of the department, Sir Hugh Sinclair, Nash has found himself with two obstructions of sorts to personal life he claims that he so deeply wants. The first is that he has shown over the years of working for Sinclair that he is a very capable operative and so likely the first man that Sinclair would think of to send on highly important, and thus certainly deadly, missions. As a result, Nash is seldom back in the UK for long before yet another problem arises necessitating his leaving. The second is that he is very much in love with, and has that affection reciprocated, by Sinclair's beautiful and independently minded daughter, Emily, a personal secretary to the First Lord of the Admiralty, Winston Churchill. Nash desires very much to marry the woman and even proposes but then another mission is needed by Sinclair and off Nash goes again. As much as Emily Sinclair would want to accept the proposal, she is determined that she will not be "a bride one day and a widow the next". To his credit, the elder Sinclair approves of the relationship and would not mind Nash as a son-in-law but then another problem comes up and ... the relationship stalls yet again.
It is prudent here to point out that while Nash is definitely annoyed at the state of his love interests, he cannot honestly say that he only accepts the missions he is given because of dedication to King and Country. Yes, he is very much a patriot but there is no doubt that Nash adores the excitement and danger that each new assignment brings him and would chafe were he to be given the time off he claims to desire.