Batman: Arkham City: Robin, for use in two challenges.
This sounds a lot like the Joker DLC that was exclusive to PlayStation 3 owners in Batman: Arkham Asylum. The Joker was only used for the combat challenges, so it's probable that's what Robin would be used for. There's still no confirmation that Robin will appear in-game.
Could it be the Nightwing figurine is a reference to another DLC, perhaps for a different retailer?