Nintendo just uploaded a surprise video detailing the anticipated 3.0 update for Super Smash Bros. Ultimate. Among other things discussed was the reveal of the returning Mii Fighter costumes for Tails and Knuckles.
These costumes debuted in Super Smash Bros. for Wii U and 3DS. In Ultimate, these can be purchased on the eShop for 75 cents USD each. Note that these do NOT come with the Fighters Pass and must be purchased separately.
The 3.0 update and all its contents, including the Mii Fighter costumes, will release tomorrow on April 17th.