Along with the return of every character that has ever been in Smash Bros., and newcomers the Inklings from Splatoon, Daisy will be a new fighter.
Director Masahiro Sakurai describes her as an “Echo” of Peach, which means, like Captain Falcon and Ganondorf or Fox and Falco, Daisy is a character that copies the basic moveset of another character. In this case, she will have a similar moveset to Peach.
Source: Game Informer Daisy Confirmed For Super Smash Bros. Ultimate’s Roster