Either he is Khan who somehow got into Starfleet or he is Gary Mitchell. Obviously this guess is based upon the writer's claim that he is an iconic TOS character which could very well be a lie.
Furthermore I guess that his character will be altered slightly, probably roughly as much as Kirk. I also think that there is a decent chance that the character will be as interesting as Cochrane in FC and not, like in the previous movie, merely a cheap way to hedge themselves against dislike from old-school fans with massive fan-wank.
I nonetheless maintain my basic criticism of Trek being not bold enough to go where no one has gone before, a structural issue that might not even be solved by the next series.