Since the games very first release on the Playstation 3 (Disgaea 3), Nippon Ichi Software have incorporated DLC content into the titles, mostly in the form of crossover characters whom never made their way into the main end-game content. The same holds true for Disgaea D2: A Brighter Darkness, with it recently being unveiled that main characters from Disgaea 2 Adell and Rozalin, as well as Ririeru from The Guided Fate Paradox are to be made available as DLC characters for this release.
Each of these three characters are now available for the Japanese release of the game for 300 yen apiece, and I assume are set to be available for the international edition post-launch later this year.