In Batman: Arkham City, there are different playable characters that are available with pre-orders. It appears they are either Catwoman, or Robin. I've seen mention of Nightwing, but I think that's for later release.

My question is, do these playable characters have an affect on the main storyline? If I have the Catwoman pack, does that change the main storyline? Or, is it different with Robin? Or, are these just typical extra characters available to use in challenge maps, and things like that?