Susan Merton is an agent with American Army Intelligence.
She will be.
When we first meet her, in the days leading up to the entry into World War II by the United States, she is a nurse, relatively new to the hospital where she works, placed in a position that is not what she was looking for but holding out hope that things will get better. She will, after her initial adventure, be inveigled into working for Military Intelligence and will in short order show those who did not believe in her that she was actually very good at it. She will have missions in multiple parts of the world starting in the Midwest of the country and moving to the Pacific, the Caribbean, North Africa, and even Russia. Along the way she will court, and be courted by, a doctor she will soon marry and start a family.
But first she is a nurse in an unhappy position. And she is a bit of a gold-digger. When being flirted on by a new doctor at the facility, she opines (to herself), "he was so good-looking that she was rather sorry he was so poor" because "good looks didn't pay for a Park Avenue apartment and her mind was clearly on such things, as well as sables". Indeed, in that first recorded adventure, she does nothing related to the cloak and dagger world but instead fixates on getting a better career position and finding a potential mate that would make having that career not so vital.
It is when she and her recently chosen husband-to-be, Dr. Tam McDuff, are sent by the Army to a base in the Mid-West that she starts on the path in the Intelligence world that will define her professionally for several years to come. And she will not limit her involvement to tracking down traitorous black marketers and foreign saboteurs. She will learn to be a paratrooper and she will become an expert in interrogation and quite a bit more.
She may have started out as someone dedicated only to finding a rich husband but she does not stay shallow for long.