Yes, Disgaea 5 will have plenty of classic characters as DLC

Flonne confirmed

Disgaea 5: Alliance of Vengeance is coming to the US on October 6 on PS4, and in true NIS, it will have lots of character DLC. It will arrive in five batches, which will feature characters from Disgaea 3, 4, and D2, as well as “other games” like La Pucelle. NIS is planning a Season Pass, which grants you Pleinair as a free bonus.

So yeah this is nothing new for the publisher, especially when it comes to this franchise, but it’s still nice to know exactly what games they will be drawing from, and that the DLC is confirmed for other territories. It also makes for a killer pickup when they produce the inevitable remastered edition with all the add-ons in tow.

Disgaea 5: New Features Detailed, DLC Outlined [PlayStation Blog]