So who exactly is this Arkham Knight? Well here are a few possibilities
#1 Hush
Now with his cameo in Arkham city I think that this is not it for his character.Now why do I think this? Well with this being the end to The Arkham game series Rocksteady would not just start his arc and leave it. But If you look closely Arkham Knight has a gun holster with a gun in it which Hush does use guns alot, also this is said to be more personal game about Batman and what is more personal then when this man knows who you are and knows almost everything there is to know about you. Same goes with the next few. anyways this theory is not as good as the next.
#2 Red Hood(Jason Todd)
If you look at both RH and AK the both share the motorcyclist look, both have the gun holster and both have a connection of sorts to Batman. Now both helmets look similar too just one has the bat ears and the other doesn't. Again this would be personal to Batman because it would be one his own sons. here is the link to the interview about Arkham Knight:
http://www.gameinformer.com/b/features/archive/2014/03/05/revealing-the-story-of-batman-arkham-knight.aspx By the way I do not know to post videos so that is why it is a link.
Or he or she can just be new character too which is most likely it.
#3 Azrael
To be more specific Jean Paul Valley. Just by the mask and that he has a gun seems to be the violent Batman that Jean was as Batman.