Super Smash Bros. Ultimate officially calls fighters with movesets closely based on another fighter “Echo Fighters.”
Echo Fighters share much of their core design with a corresponding “original” fighter, but they are presented as distinct characters and can have differences in animations, attributes, timing, or individual moves. Ultimate introduced the official English label and marked these characters with an epsilon symbol on the fighter-selection screen.
Examples include Lucina, based on Marth; Dark Pit, based on Pit; Daisy, based on Peach; Richter, based on Simon; Chrom, based on Roy; Dark Samus, based on Samus; and Ken, based on Ryu. Players can display Echo Fighters separately or stack them with their corresponding fighters.
The term does not mean a character is merely an alternate costume. Echo Fighters occupy their own playable slots, while costumes such as the Koopalings for Bowser Jr. use the same moveset and are not separate fighters. “Mii Fighters” and “Assist Trophies” are entirely different gameplay categories.