Throughout the Nintendo's Super Smash Bros. Ultimate Direct, it was announced that any player who pre-orders Ultimate will receive a completely free Piranha Plant as a new fighter.
According to the announcement, Nintendo will only be providing this opportunity for a limited time only, players will have to pre-order and claim their Nintendo Gold Points before January 31st, 2019 to receive the fighter. "If you purchase Super Smash Bros. Ultimate and claim your My Nintendo Gold Points by January 31st 2019, you can receive the fighter Piranha Plant as free downloadable content!"
Once players have claimed their Nintendo Gold Points an email with the fighters code attached to it will be sent to your Nintendo Account email address. After the Nintendo Gold Points have been added to the account, the fighter code will be sent out within 72 hours of activation, all codes will last until June 30th, 2019. For more information visit the official Nintendo pre-order listing here.
How to get the early purchase bonus: