Batgirl will be available for PlayStation 4 and Xbox One starting on the 21st July. As the PC version of the game itself was withdrawn from sale, there is no word on when Batgirl will be available for PC players.
The Batgirl DLC will be available for $6.99 / £5.79 or as part of the Season Pass. Season Pass holders will also have the opportunity to get the DLC a week earlier, from the 14th July.
The story is set prior to Arkham Asylum and allows Batgirl to co-operate with Robin for take downs. Batgirl of course appears in the main Batman: Arkham Knight story line as the Oracle
The Batgirl DLC was developed by Warner Brothers Montreal, the same studio that developed Batman: Arkham Origins. Warner Borthers did confirm that future DLC will be developed by the game studio responsible for the game itself, Rocksteady.
Batman: Arkham Knight is available for PlayStation 4 and Xbox One, and PC once Warner Brothers returns it to retail.