Rex Bennett is an agent with the American government.
Looking at the title of the first recorded adventure of Bennett, the use of the term 'G-Men' would make one believe he is a member of the FBI since that is the moniker they are most known by. However, there is nothing else to point that way. When we first meet him, he has his office in a federal building in Los Angeles; on his door is the title "Rex Bennett, Special Investigator". There is no indication that it is part of the Bureau's L.A. offices. Add to that the fact that his second adventure takes him far from California to the heart of Africa and it looks like he is working for something else inside the U.S. government.
Bennett is likely in his mid-30's on that first adventure. He is a ruggedly built man with a solid firm looking face, not exactly handsome per se but by no means unattractive, especially considering his tall physique and demeanor. He is quick to smile and inclined to be amiable by nature but his civility can disappear instantly should people decide to be unpleasant. As he demonstrates no hesitation to use his fists in a scuffle or to pull his revolver when fired on, Bennett can prove he is no easy mark.
The facts that he has his own office, with his name on the door, and a secretary to announce visitors, shows he has been around awhile and risen more than a couple of rungs in the career ladder. Adding to that is the fact that he is on a first name basis with agents of foreign governments. While we do not learn anything about his previous experiences, there is no doubt that he not only had a good number of them but that they also ended in his favor.