"Davvie" McLean is an agent with the U.S. Naval Intelligence.
She is also a nurse. And a housewife. And a pretty good detective although being a female in the late 30s, it would be a very strange man who would think to admit that lady could do much with her head other than look pretty. But she can think with hers and luckily for the Navy she does it well.
McLean is an officer in the Naval Nurses Corp. [It would not be unexpected to think that McLean was an officer in the Navy WAVES (Women Accepted for Volunteer Emergency Service) as most people would think WAVES when talking about females serving in the Navy in the 30s-50s but McLean was in the service in 1937 and had likely been so for a year or two beforehand and the WAVES did not come into existance until 1942.]
She is not, when the first recorded adventure takes place, at all involved with Naval Intelligence and would have likely frowned at you if you suggested such a thing. Her desire was to tend to the infirm and that is where we first meet her in 1937. Stationed onboard a hospital ship, McLean was one of those aiding in the evacuation of dependents from a China in the midst of war with Japan. She is single in that escapade but is quite close to one of the doctors on the ship and it is when that doctor becomes the chief suspect in a murder that McLean shows she has a special talent for digging for the truth.
It is in the second adventure that she comes to the attention of the intelligence community when, in Cuba with her now husband doctor, she assists in solving another murder (all of her adventures have a dead body or three lying around) and that one involved codes and clandestine meetings and all the things operatives love so much. Having proven her intelligence and her tenacity, she is a perfect candidate for further use by them in the last two tales we have of her.