An Echo Fighter is a term used to describe characters who are considered an almost complete clone of another character. sharing essentially all of the main attributes of the original character (such as speed, jumps, KO power and weight), to the point where you can play the two identically even in competitive play. Echo Fighters always have unique taunts and victory poses, and may receive additional changes (such as other animations or some gameplay adjustments) if necessary. They are featured in Super Smash Bros. Ultimate and all of them are unlockable.
For example, Daisy plays virtually identically to Peach, sharing every single move and attribute. However, she has various different animations and visual effects. Lucina's gameplay and animations are all extremely similar to Marth's with only the sword properties being different.
Similar characters such as Dr.Mario or Pichu, who have major differences in their attributes and move properties, are not classified as Echo Fighters. Characters such as Alph and the Koopalings share every animation and only differ in model and name, so they are classified as a Pallete Swap.
Characters who are Echo Fighters are denoted by the Epsilon (ε) letter from Greek alphabet in the International version, while in Japanese version is denoted by the Prima (').