Notes about alternate characters:
Similarly to Echoes in Ultimate, Gaiden is host to alternate characters. Unlike any Smash game, however, Gaiden has three different types of alternate characters:

-Echoes: These characters play closely to the character they are based off of; more often than not, an Echo is an alternate form or iteration of the base character. Examples include Dark Link and SA-X.
There are certain exceptions, despite said exceptions playing similarly to their bases. Dark Meta Knight and Dark Pit are exempt, for example, due to being entirely separate characters in their home series with their own unique traits, as opposed to one-off entities or constructs. Young Link is exempt due to being a separate character in Smash since Melee, and his height making a difference in playstyle. Young Ganondorf is exempt due to being unique compared to the other iterations, and to contrast Young Link.
-Hidden: These characters play drastically differently from the character they are hidden behind. Like Echoes, Hidden characters are often alternate forms or appearances of their base characters. Examples include Shadow Mario and Masked Marth.
There are certain exceptions. The four Links are exempt due to actually being different characters (Young Link and OoT Link are separate due to different playstyles). Dry Bowser is exempt solely due to personal preference.
-Boss: This is a combination of the two aforementioned types. These characters have drastic stat or attack changes that make them unviable for competitive play. Used for fun. Examples include Metal Mario, Ganon, and Giga Bowser.