Sazh is easily the sixth ranger, he only serves as the foil to Lightning for the first chapter or two. Past that, he blends with the background, exception being part of the dyad with Vanille. Overall, I'd go:

Hero = Lightning
Lancer = Snow (Especially during the early chapters)
Big Guy = Fang (Less obvious, but she rules in Commando)
Smart Guy = Hope (Excels in magic, though really he's not that smart during cutscenes)
Chick = Vanille, obviously
Sixth Ranger = Sazh (Serves as lancer early on and joint hero during Vanille/Sazh sections)