Joe Rodriguez is an agent with American Military Intelligence.
It is the mid-to-late 60's and the United States government is fighting hard to stem the feared 'domino principle' of communism where the loss of one country would lead to the downfall of a neighboring nation and then the next one and so on. Fighting desperately to prop up the South Vietnamese regime, Americans were finding it nearly impossible to tell the good guys from the bad guys and in the southern capital of Saigon, the infiltration of North Vietnamese spies was intense. Throw in a whole lot of drug smuggling and black marketing and rising gang activity and you had a true mess.
That made it important for the G-2 people to have people on staff who had more than an inkling into how spies and thugs, smugglers and pushers operated so they could have any hope of keeping a handle on things.
Enter one of those people well versed in both intelligence work and good old fashioned police detective work - Lieutenant Joe Rodriguez, a 27-year-old man in the Army Reserves who is pulled back into active duty to serve a tour or two in Southeast Asia. Instead of working with the Military Police, however, Rodriguez is a part of the Intelligence operation there, likely from having been in it during his normal time in the service (conjecture).
We actually meet Jose Juan ('Joe') Rodriguez a couple years before he finds himself relocated to the other side of the world. Our introduction to him was as a "rookie officer of the law" who was (and this is a tad confusing) pleased to be able to dress in plain clothes while responding to calls to investigate crimes. He was spending some quiet time in the squad room of Division 6, doing homework from the UCLA psychology class he was auditing in his spare time when he got the call to investigate the first crime we watch him handle, a naked male corpse in a swimming pool at an apartment complex.
Rodriguez would answer several more calls to investigate suspicious deaths before being pulled back into military service to work during the height of the Vietnam War and the cases/missions he would be given would morph into those more in the cloak-and-dagger mode.
He is described as "a slender young man with Spanish features" possessing a face with a "classical mold .. almost in the lady-killer tradition .. saved by a strong flatness in the set of the eyes".