I read a blurb that made it seem like a side-activity. basically, you see cats around, and when you 'interact', you take control of her. They say things like "seamlessly interwoven with his main story arc" and "Catwoman's adventure compliments Batman's central role" and "Complete with her own storyline in the main game."
Doesn't sound like it will be like the Ashley section in RE4 or required. It's kinda like the Ada Wong Separate Ways, where it tells the same event from a different perspective. I don't know.
I like that movement around the city changes. Looks like ass creed.