Is Niki Sanders from the show Heroes really dead? Or does she show up later?

I'm at the beginning of season 3 of the show. Niki was left in the burning building and the cheese-a** copycat girl survived. I've seen Tracy and apparently they were triplets...but I don't like Tracy as much as Niki. I miss Niki! And what the hell happened to Jessica? Does she stick around with Tracy now?