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http://www.gameinformer.com/b/features/archive/2014/03/12/who-is-arkham-knight.aspx
Summary
-Arkham Knight's suit is black with red highlights and has an "A" symbol on his chest
-Arkham Knight uses silenced pistols
-Arkham Knight's suit is made up of armor and has multiple belts and pouches
Theories
Arkham Knight could be a whole new character or could be an existing character in the Batman universe including:
1. Red Hood - The color scheme is similar to the Arkham Knight character and Jason Todd is known for wielding guns, also a specialty of Arkham Knight. Also, Jason's story has never been established in the Arkhamverse which could play a significant role in the game.
2. Batman's Brother - From the trailer, we hear Thomas Wayne's last will and testament to Bruce. This could foreshadow that there is more to the Wayne family than meets the eye.
3. Wrath or Prometheus - Both characters have similar back stories to Bruce Wayne. All three of these characters lost family members at a young age, seeking vengeance. However, as Batman refuses to kill, Wrath and Prometheus are willing to use deadly force. These characteristics are similar to Arkham Knight.
4. Azrael - Not only does his name start with the letter "A," just like the logo on Arkham Knight's chest, but he also warns Batman at the end of Arkham City that Gotham will burn. This prediction seen in the next installment.
In the explosive finale to the Arkham series, Batman faces the ultimate threat against the city he is sworn to protect. The Scarecrow returns to unite an impressive roster of super villains, including Penguin, Two-Face and Harley Quinn, to destroy The Dark Knight forever. Batman: Arkham Knight introduces Rocksteady's uniquely designed version of the Batmobile, which is drivable for the first time in the franchise. The addition of this legendary vehicle, combined with the acclaimed gameplay of the Batman Arkham series, offers gamers the ultimate and complete Batman experience as they tear through the streets and soar across the skyline of the entirety of Gotham City. Be The Batman.