Oh, for pete's sake.
LET'S DO THE THING:
Echo Fighters are characters whose hurtboxes are exactly the same as the character they are based off of. They also have the same attack animations with minor exceptions. They can possess different weights, attack power and properties, and literal model differences, but they are for the most part exact copies of another fighter. Another important detail is that an Echo Fighter is a UNIQUE character.
Fox/Falco: At the time of their first appearance, Falco was a CLONE AND AN ECHO FIGHTER, as he possessed all of the same animations and model size as Fox, had a slower running speed and faster fall speed, as well as a higher jump. Now that Falco's jab animation, forward smash, neutral aerial, forward aerial, and down special have been substantially changed giving them drastically different animations and hitboxes, he is NO LONGER AN ECHO FIGHTER
Captain Falcon/Ganondorf: At the time of their first appearance, Ganondorf was a CLONE AND NOT AN ECHO FIGHTER, as he was a resized Captain Falcon model. Over time, his model and animations have continued to change, and with Smash Ultimate, he has been given entirely new smash attacks atop his very different smash special animation. Ganondorf was NEVER AN ECHO FIGHTER
Link/Young Link: At the time of their first appearance, Young Link was a CLONE AND NOT AN ECHO FIGHTER as he was a resized Link model. In Smash Ultimate, he is neither an Echo Fighter to Link nor Toon Link, as his model is a different size, and his moveset matches Toon Link, but not Smash Ultimate Link. Young Link was NEVER AN ECHO FIGHTER
Marth/Roy: At the time of their first appearance, Roy was a CLONE AND AN ECHO FIGHTER, as he possessed all of the same animations as Marth, had a slower running speed and faster fall speed, along with fire properties and a different "tipping" mechanic. Now that Roy's jab attack, forward tilt, forward smash, forward and backward aerials, and counter animations have been substantially changed giving them drastically different animations and hitboxes, he is NO LONGER AN ECHO FIGHTER
Pikachu/Pichu: At the time of their first appearance, Pichu was a CLONE AND NOT AN ECHO FIGHTER, as he is a resized Pikachu model. Because of this size difference, Pichu was NEVER AN ECHO FIGHTER
Mario/Dr. Mario: At the time of their first appearance, Dr. Mario was a CLONE AND AN ECHO FIGHTER, as he shared a moveset and model size with Mario despite having different attack and weight properties. Now that he has a different down special and down aerial, Dr. Mario is NO LONGER AN ECHO FIGHTER
Ness/Lucas: This is a stupid comparison, as Ness and Lucas have very different attack animations and properties despite being similarly sized. Lucas WAS NEVER AN ECHO FIGHTER
Marth/Lucina: Lucina possesses all of Marth's attack animations with one exception, being her down aerial having a different sweeping animation that goes from back to front rather than front to back. The only substantial difference between the two is Lucina's lack of "tipping." This means SHE IS AN ECHO FIGHTER
Mewtwo/Lucario: This is an even dumber comparison, as these characters share a similar neutral special attack and ABSOLUTELY NOTHING ELSE. Lucario WAS NEVER AN ECHO FIGHTER
Pit/Dark Pit: Dark Pit possesses all of Pit's attack animations, and his Electroshock Arm is an aesthetic alteration with different attack properties. This means HE IS AN ECHO FIGHTER
Peach/Daisy: Daisy possessed all of Peach's attack animations, with one apparent exception, being her up aerial, which Peach might also have. Her attack properties may be different, as well. This means SHE IS AN ECHO FIGHTER
Based on this, Echo Fighters must share a similar model size and have extremely minimal animation changes, while still possessing the same moveset at the original fighter with tweaked attack attributes. The best comparison I can think of is putting a Shadow the Hedgehog skin on Sonic, making him more slippery than the former, and making all of Sonic's blue attacks yellow.