It's not "real" in that, as it's portrayed, it doesn't actually exist. However, I have no doubt that people like Sam Fisher (or Jason Bourne) does exist in reality. "Black Ops" are very, very real, and just as other governments have agents in "deep cover" who live amongst a population and are activated as needed for special operations, I see no plausible reason to believe that the USA doesn't likewise have exceptionally-trained agents around the world who are used for highly sensitive (and perhaps questionable) operations.
Of course, such people would be "off the books" assets who would be disavowed if they get caught, and their activities would only have sanction on an unofficial level to preserve deniability. How someone gets "recruited" into that world is a haphazard guess at best.
Chuck Barris (Host of The Gong Show) was reputed to be a CIA agent who was trained to do many things including assassination. The CIA denies this is true. You can watch Confessions of a Dangerous Mind and see what you think for yourself. I'm sure people who have the right "talents" or otherwise get "noticed" by an alphabet agency might get "recruited" into the life, but I'd never expect someone actively looking for it to be picked as it would automatically appear to be suspect.