Super Smash Bros. Ultimate's first DLC character, Piranha Plant, is coming very soon. The new fighter is slated to arrive this February, and unlike Persona 5's Joker, it will be included as part of the game's Fighters Pass. However, early adopters will be able to receive the character free of charge – but your time to get it out.
This was done automatically if you purchased the game digitally through the Switch Eshop, but if you picked up a physical copy, you'll need to register at the plus button while highlighting the game's icon in the Switch menu, then select the My Nintendo Rewards option.