Jeff Shannon, the Eagle, is an agent with American Military Intelligence.
As described in one of the adventures, while he often while traveling listed his occupation as "correspondent", implying he was a reporter, he was in fact "the ace agent of the G-2 branch of the United States Army Intelligence Service". Usually he answered to the Secretary of War but on occasion he could be assigned temporarily to the State Department.
Shannon is described as being in his late 20's, tall, lean, with brown hair and blue eyes. He is perpetually impeccably dressed in a suit and tie - the shirt being a constant white and the tie always in good taste.
Professionally, Shannon is known the world over, "from Tokyo to Berlin; known and hated - and feared", especially in the Intelligence departments around the globe. His nickname of the Eagle is also common knowledge, though why he is called that is not known. Even the bad guys know of the Eagle though at least some of them have not been privy to the connection of Eagle to Shannon.
Before becoming the Eagle, "he was once more plain Jeff Shannon, crack secret agent of G-2, unofficial operative of the F.B.I. He had entered the work with several years of training in the State Department and the code offices." He took the time to study existing photographs of enemy agents and learn all the tricks that had been used by them. He did not take on the persona of the Eagle until "he was as well versed in this grisly business as any man alive".
Shannon is a very cool operator, relaxing and lounging whenever he can. He can often be seen lying back in a chair, his feet resting on some nearby object, a cigarette in his hand, showing he has hardly a care. When action is required, however, he is ready and able at the drop of a hat or the drawing of a gun.
Shannon is, among other things, a master at breaking codes; "he could read ordinary codes almost as easily as he could English and harder ones broke down under his diligent attacks". Apparently his expertise with languages is also excellent for he has no trouble communicating with the citizenry no matter where he goes.
Noteworthy observations:
"When a problem so difficult as to defy solution presented itself, the Eagle usually whistled."
"[Shannon] was sure there was always a loophole somewhere; that the best of spies leave crevices in their plans where he might get a fingerhold and pry open the whole rotten affair."
"With right on your side you can't lose, if you've the courage to back up your convictions."
"I have never called for help yet. If help presents itself, very well. I use it. If not, I fight my own battles."