Nightmare Robin is actually Bruce Wayne's son Damian, who got infected with the vampiric curse and is a little miffed at being one of the undead. At least he has two large knives and a lot of bleeding-edge training to compensate. Atomic Joker is a bit more lighthearted, what with being a head in a jar who is trying to rebuild civilization in his image.
The September release overhauled the launcher, improved the new player experience, polished many of the map visuals, tweaked animations, and added a Crime Alley map where champions can test out skills against the computer.
You can check out Atomic Joker and Nightmare Robin in the gallery below and in the videos after the break.