Phyllis Bowden is an agent with the CIA.
She will be, by the third recorded adventure. In the first one when we are introduced to her, she is not yet affiliated with that organization because it does not yet exist, the year being 1944. She is not even officially connected with American Military Intelligence though she will be working for them in the next adventure. Back to that first one, she is a secretary at the American Embassy in London who is about to find her career and her life changed drastically because of the drastic change in career of someone else at that facility.
Bowden is in her mid 20s and has been with the War Department in a clerical position for several years, the last eight months working at the Embassy as the secretary to the Assistant Military Attache - make that the Acting MA since the man who just held the position was arrested for espionage (he is the person alluded to above). That man's unfortunate plight will be what will get Bowden to look into the matter, off the record, and that will soon lead her to finding her own line of work change.
One point of clarification here: The Military Attache section of the Embassy is almost by definition a part of Military Intelligence and since Bowden is the secretary to the person in that position, she really is in MI but not really. Her role is that of a stenographer and typist and filer, not anything involved in intelligence gathering, analysis, or reporting. Up until she decides it needs to be. Once her determination to solve a mystery and right a wrong pushes her into action, she begins the transition from a secretary to an assistant to an operative on her own.
When we first meet her, Bowden is unattached (that will change) who constantly receives letters from her older sister to return to the States to help care for her ailing father. Her heart cracks a little with each new request but she is determined to play her part in the war effort - if the men her age are putting their lives at risk to fight the Nazis, how can she stay at home safe. When the War ends in the first adventures, she finds that there are always new enemies that need fighting.