The latest addition to Super Smash Bros. Ultimate is Byleth from Fire Emblem Three Houses. They will be available on January 28, but you can get your hands on five new Mii Fighter costumes before that, on January 20. Additionally, director Masahiro Sakurai confirmed that six more DLC characters will be added to the game.
Super Smash Bros. Ultimate Adds Byleth & Cuphead - GS News Update
And more characters are on the way!