Just want to say, I didn't read every single comment on this thread. This might have already been mentioned, or it might just be painfully obvious to everyone. I'm kinda new here though, so don't kill me TOO much.
"The world became ours, and WE lost it. Nothing, it appears, is more challenging to the souls of MEN than victory itself."
That was the back of WoK. To me, that seems like pretty indisputable evidence that it is a human on Roshar and not a spren or something.