Agent Nine is an agent with the Bureau of Investigation.
In the description of the organization to which Agent Nine belongs, it is written as the FBI but in fact, the recorded adventures took place just before the FBI came into existence. Its predecessor, known then as the Bureau of Investigation of the Department of Justice, had been operational since 1908 and was an integral part of the Department. It had yet to become the autonomous agency it has been for the past 80 years. In fact, the two adventures that exist of Agent Nine were released the same year that a still youthful J. Edgar Hoover, fifth director of the BoI, would be authorized to reorganize that bureau into the Federal Bureau of Investigation.
Agent Nine's real name is Bob Houston. When we first meet this young man he is a "youthful clerk in the archives division of the War Department". He entertained thoughts of a future in the Bureau but had not yet completed college so knew that he would not be accepted. Wanting, however, to serve his country in any capacity he could and, of course, needing some form of income, he applied to and was accepted by the War Department (prior name for the Department of Defense) to work in the archives.
It is because of his job in those archives, a job which routinely entailed cataloging and storing all manner of plans and schematics for new armaments, that would get him connected to the BoI through his Uncle Merritt.
Merritt Hughes was a "crack agent of the Department of Justice" and a hero and mentor of Houston. Since the last names were different, it is likely that Houston's mother was a sister to Hughes for though their surnames were not the same, it is stated that they looked close enough to be considered brothers (Hughes being several years older). It would be Hughes who, needing an "in" to the archives to solve a worrisome case, grabbed his nephew for help and in so doing, pushed him further down his career path.
Agent Nine is described as around 5'10" tall weighing about 170 lbs. He has light hair and blue eyes and can be considered around 20 years of age.