Like others said, they are "characters" in the world technically but don't ever expect to see or hear from them as they represent the player base. And so far as the opening movie goes, they represent us legacy players there.

The battle you see in the cinematic was an actual in-game event that took place in the final day of version 1.xx. I remember very well the hours spent of us legacy players gathering and watching the red moon crashing down and the Imperials invading with a massive war breaking out all across the game world. *sigh*. Even at the end of the cinematic you see them being teleported to some remote place... us legacy players actually go through that. When we started in 2.0 with our old characters, we appear in the same location and we even get the same armored black tail chocobo you see the warrior ride away on in the end.

So yeah basically, their legacy players.