The three main characters on Mindhunter are based on real people, though they aren't exact copies. The two FBI special agents from the bureau's Behavioral Science Unit are Holden Ford and Bill Tench, played by Jonathan Groff and Holt McCallany, respectively. Those two men are based on John E. Douglas (who co-authored the book on which Mindhunter is based) and Robert K. Ressler.
Douglas, now 74 years old, is one of the first criminal profilers in the United States. After serving in the Air Force, he went to work for the FBI in 1970 - first as a sniper in the Swat