Lieutenant Armstrong is an officer with U.S. Army Intelligence.
His rank is used where a first name would normally be used because we never learn his first name. He certainly has one (the Army requires it) but we are never told what it is. We only known he is a lieutenant in the American Armed Services.
We also know from his depictions that he is male, Caucasian, and somewhere in his late twenties. He has short (required) hair, brown in color. He is good looking and in quite good shape in the "lean and mean" variety.
There is not much more that is told to us largely because he is not around very long but a couple of things can be inferred. Since this short series takes place just before the Americans got into World War II and historically advancement was not particularly fast then, he likely has been in action for half a decade.
He is quite brash and impulsive despite his likely experience because he dives into danger (literally as well as figuratively) with nary a hesitation (or much thought) and he runs afoul of the bad guys as a result. He is resourceful and self-sufficient because he gets himself out of his problems, usually by himself. He is not oblivious to his impetuosity because he on a couple of occasions has to admit to himself that he got "lucky" to have survived something he did on the spur of the moment.
We also know that he is thought well of because he always works alone and his field of operations is quite wide. Officers not trusted or respected get desked and a desk is not something Armstrong bothers with.