Hm, odd! Based on what we know, I've got some guesses.
Assuming that the "Pony hair" alt in Smash is indeed this new character (which seems likely to me), that also means that she has to fight very similarly to Joker. It was found to be an alternate costume, not even an Echo Fighter, which means that the two of them have to fight almost identically. Also, I find it hard to believe that they'd make her an alternate costume for Joker in Smash just because she's a new character in P5R - she'd have to fit the role.
So that makes me think she must be playable in some capacity! I doubt it's just a new character like Marie, because there would be no reason for her to fight just like Joker. Based on that, I can come up with three possibilties:
-She's a female protagonist! This seems to be countered by the trailer, because her pin says she's a first year, but Joker is second year (in fact, no member of the party is a first year at Shujin!) She also says that she doesn't agree with how the Phantom Thieves are doing things. A female protagonist doesn't seem impossible to me, despite the oddities - maybe her story ends up significantly different from the original male protagonist's, so it's like a new story? That might explain some of the differences - for example, maybe she knows of the Phantom Thieves because she isn't a part of the team when they start?
-She's the star of a new mode like P3FES's Answer! Some new mode where you play as her and... I don't know, look into the Phantom Thieves? I would think that the star of a new mode would fight a bit differently from Joker, but maybe not! Based on the fact that her words indicate the Phantom Thieves are still around, I guess it could be a mode you do after beating the game, but its story takes place at the same time as the main story, rather than afterwards?
-It's also possible that "Persona 5 The Royal" isn't an enhanced version of P5 at all! And in that case, she'd be the main character of whatever this is. Since 4's enhanced version was "Persona 4 The Golden" in Japan, though, it makes sense to me that this is an enhanced version of 5.
It's impossible to know for sure without more info, but I think it's one of the first two possibilities for now. Looking forward to hearing more!