I have only three speculations:

1) The Warrior of Darkness is somebody we've never met, but they used the cinematic guy to conceal his identity.

2) The Warrior of Darkness is someone we know, but they're concealing that person'a identity.

Or, my most farfetched speculation, 3) The Warrior of Darkness represents a faction to and from which we can switch as we wish, giving us the thing many people have been asking for: open world PvP, among other things.