According to the latest issue of Dengeki PlayStation magazine (helpfully translated by Gematsu), Disgaea 5 stars a young demon boy named Kiria who is seeking revenge against Demon Emperor Void Dark. Kiria is joined on his quest by Seraphine, the princess of the Gorgeous Underworld, and Usaria, the unstable queen of "Rabbit Rabbit World" who goes "berserk if she does not eat curry."
As for why the game is exclusively coming to the PlayStation 4, NIS president Sohei Niikawa explains: "We thought about going multi-platform with PlayStation 3, but doing so would've made the PS3 the standard," Niikawa said. "Since we're making it, I want to be able to offer something that could only be done with the PlayStation 4." As an example, Niikawa points out that Disgaea 5 will be able to feature 100 characters on screen simultaneously, where a PlayStation 3 version of the game could only handle 10.
Disgaea 5 is slated to hit the PlayStation 4 in Japan in the Spring of 2015. There's no word on an English-language release as of yet, but given the series' history it seems likely the strategy roleplaying game will eventually make its way to the West.