I guess it depends on what you mean by "realistic". There are definitely guys like Sam, so even though the situation is unrealistic his actions are pretty believable. Maybe the better question is "Who's unrealistic?" Aiden is definitely a character that is pure fiction. A 17 year old mastermind that can't be touched and manipulates everything around him? Sure.
Bennett was pretty dumb too, but he's played such a minor role that I don't count him.
Morello is also a character that could only exist in fiction. Handsome, powerful, dangerous, wealthy, criminal, with honor? Yeah...ok!
But that's ok, it's only a game and not every character has to be 100% realistic.