I said it once before, but if "Jane" ends up being a no-show in Smash Bros Ultimate by the end of its post-launch development cycle, she'll go down as one of, if not the biggest development mystery in the series, unless Sakurai discusses it later.
That said, I don't expect anything related to "Jane" anytime soon. Maybe later this year, but definitely not anytime soon (Maybe during E3 or the second half of the year following E3). If more Persona x Smash Bros. related stuff happens at the concert or sometime before E3, then color me surprised.
At this point in time, I am far more inclined to believe "Jane" is either a dummy file or a newcomer unrelated to the Persona series that is built from Joker's development assets than believe her to be the mystery P5R girl. Of course, my gut could be wrong and "Jane" is indeed the mystery Persona girl.
That's my two cents on "Jane", you may return to your regularly scheduled forum posting.