Because this is such a well known and documented photo, we actually know the agent’s name was Robert Wanko.
Looking up his obituary provides at least a basic overview. He was a US Army Investigator for the Military Police. I don’t know what that looked like in the 1960s-70s, but in the modern day to be bumped up to an Investigator there are some higher standards than a normal MP, and a requirement of a Secret level clearance. Then he got out of the military, got a college degree, and then got hired by the USSS. He worked on both Carter and Reagan details.