Atlus USA announced on their Facebook that the Adachi DLC for Persona 4 Arena Ultimax will be offered for free for the first week after the game’s launch. This applies only to North America, as far as we know.
The sequel to Persona 4 Arena is slated for release at the end of September of this year for the PlayStation 3 and Xbox 360. Let us know if this will get you to buy the game during the first week in the comments below.