Robin and Catwoman aren't the only characters to make an appearance in Batman: Arkham City. A deeper look into the full achievement list on PS3Trophies reveals that Nightwing will not only make an appearance, but will also be a playable character in Arkham City.
There are two specific achievements, "Nightwing Revenge" and "Campaign Nightwing", referencing the "Nightwing Bundle Pack", in which players must obtain a certain number of medals in both the original Arkham City and Nightwing Bundle Pack, as Nightwing.
For those who are unfamiliar with DC, Nightwing is the name adopted by Dick Grayson (Robin), after he graduates from the position of Batman's sidekick.
Batman: Arkham City is set to hit store shelves on October 18 for Xbox 360, PlayStation 3, and PC, but there has been no information regarding Nightwing DLC's availability or price.