A look into the achievement list of the downloadable content planned for Batman: Arkham City has revealed another playable character. In addition to Robin and Catwoman, Nightwing (an older, saltier Robin, with a cooler outfit) will also make an appearance.
The full achievement list can be seen on PS3Trophies (via Joystiq). The "Nightwing Bundle Pack" references two achievements: "Nightwing Revenge" and "Campaign Nightwing." To get the achievements, you'll have to obtain a certain number of medals in both the original Arkham City and Nightwing DLC ranked maps and campaigns.
There is yet no mention of the Nightwing DLC's availability or price.
Batman begins his Arkham City crime-fighting on the Xbox 360, PlayStation 3, and PC on October 18.