This made me want to go looking for who it is, so... Spoiling it for myself. It's not The Joker, it's not Hush, it's not Azrael, it's Jason Todd. Or it might not be, I still don't know. The Batman Wiki site has already added Arkham Knight's name under one of the characters aliases. We'll see. I'm not sure what significance that really means to me personally if this is who it is.
I haven't looked up anything on the matter but this is who I always figured it would be. Seemed pretty obvious.