https://www.polygon.com/2018/11/1/18051614/super-smash-bros-ultimate-spirits-mode-characters

Super Smash Bros. Ultimate’s newest feature is called Spirits Mode, Nintendo announced during today’s Direct dedicated to the upcoming Switch game.

Spirits are power-ups based on other characters from many of the series represented in the game. There are primary spirits, which can include anything from Cappy of Super Mario Odyssey to Rogue from Sonic the Hedgehog to Hal “Otakon” Emmerich of Metal Gear Solid. Only one primary spirit can be assigned to a player at one time.