Rosalina hails from the Super Mario Galaxy games as a protector of the cosmos and a mother to the Lumas. During the Mushroom Kingdom's centennial Star Festival, Bowser stole the stars that powered her Comet Observatory, which is the home she built for the Lumas, so she entailed the help of Mario to get them back.
Rosalina joins Smash Bros. as a newcomer this time around, and uses one of her Lumas for extra help. She's tall, which gives her a larger hitbox, and quick on her feet, allowing for great mobility. Rosalina is unique in that she acts as a "puppeteer," which is by spawning and controlling a Luma from up close or from afar. The Luma has its own set of attacks, so this allows her to team up against her opponents with it.
Text by AbsolPowers and Smashedpotatoes