You haven't read the books? I had the impression you did.

Anyway, it's the "real" Cat, although her personality isn't exactly the same... Although it's hard to say how much of that is due to her just being different, and how much is the result of having everyone you know and love brutally slaughtered during what was supposed to be a happy occasion.

The main difference is that this Cat is no longer nice(and kinda stupid and annoying, in my personal opinion), but vengeful and emotionless. As for how, she was found in the river by Dondarrion and his band of men, and the Red Priest whose name I forgot was able to revive her. Though there were complications I don't particularly recall, but this is about the gist of it.