This may be old news to you, but in case you didn't know, Rosalina has been leaked as an assist trophy.
In case you don't know who Rosalina is, it is the Star Princess person from Super Mario Galaxy. (Sorry, I have never played SMG)
She apparently shoots out star pieces, and if you pick them up, it heals you.
The footage was leaked from the demo of the game.
So it wasn't really leaked, just not announced on Dojo.
So, if you are wondering if Rosalina is a character, sadly, she isnt.