The Dragon Commander is the Human representative on the council.
But... but... from what I've seen in Divinity 2, the Dragon Knights seem to drift away from their humanity and embrace being a dragon over being a human. The humans around the Dragon Knight certainly take that attitude - especially in the expansion.
So why would a Dragon Knight represent humanity?
A) Because his father was a king, he was a prince, presumably trained in leadership,
B) He's the leader of an alliance in a war
C) He's young enough to know his father, not hundreds of years old like the Dragon Knights you see in Divinity 2, who have outlived all their family and friends many times over, so he probably would still be tied strongly to his humanity.
D) We didn't really have much time with actual Dragon Knights, did we? We had 5 minutes with Talana, 10 minutes with Lovis the imprisoned spirit, 5 minutes with the imprisoned and angry Amdusias, and the rest of the game was with Ygerna who was impersonating a Dragon Knight.
