Fighter numbers are a method of identifying and sorting characters in Super Smash Bros. Ultimate.
As explained by Masahiro Sakurai, the numbers order characters based on "the order in which they joined" the series. Here is the list of how newcomers from each game are ordered.
The Fighter numbers are used to order characters on Ultimate's selection screen and promotional material. The only major in-game usage of the fighter numbers is that Echo Fighters, due to not having their own fighter number, can be optionally stacked onto the character they are based on exclusively in Vs. Mode, Special Smash (except for Smashdown), Tourney, and Online modes except for Quickplay.
A similar system is used for ordering stages on Ultimate's selection screen, where all stages are ordered by when they first joined the series (with the exception of stages shared between both versions of Super Smash Bros. 4, which are mixed in with stages from exclusively Super Smash Bros. for Wii U)
It is possible to use similar rules to construct fighter numbers for other games in the series. However, because Echo Fighters share number with their parent by these rules, and the clone status of some characters has changed between games (for example, Falco in Melee would be 07ε), many characters would not keep the same number across the series as a whole. As a result, the system is generally not portable across games.
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