The leader of the Phantom Thieves will be the first of five DLC characters to come to the game. Each of these upcoming fighters is completely new to the Smash Bros. series, and will come with a stage and song reflecting the games from which they originate. No gameplay or screenshots just yet on how that will translate in Joker's case, but expect lots of demons and/or high school drama.
That said, I will put money down right now that his Final Smash is a transformation into his persona. Or maybe an all-out-attack - it would be great to see the rest of the core Persona 5 cast make an appearance, even if it's just in a final smash animation. And you just know they're going to add Last Surprise for the music.
Joker will be available to purchase separately when he arrives (no word on exactly when that will be), or as part of the fighter pass. But don't worry, you've got plenty of time to enjoy Super Smash Bros. Ultimate's 70 other fighters until then. After all, the game is out in just a few minutes, on December 7.