A clone is a character whose moveset, animations, and general properties are mostly derived from another character, as opposed to being unique to them. All clones have some notable differences from their original, ranging from different animations (such as with Daisy) to fully unique moves (such as with Chrom) -- thus differentiating them from alternate costume characters such as Alph -- but on the whole, they will generally be very similar to their parent, to the point where a casual player might play either in the same way. However, this is not to say that playing a clone like their parent will be successful: Roy may be a clone of Marth in Melee due to sharing all his attacks and animations, but as his movements and hitboxes are all altered, his optimal playstyle is completely different.
Clones are a common inclusion in various fighting games, as it is significantly less expensive in time and resources to develop a character using another character as a base than it is to do everything from scratch, while still potentially forming a character of unique playstyle and fanbase. However, despite being easier to make and thus resulting in a larger roster overall, fans are quick to show disdain for clones, perceiving them as stealing resources from potential unique characters.
The term "clone" can also be used to refer to individual moves. For example, Charizard's Flamethrower is a clone of Bowser's Fire Breath, as even though the characters are not related, they are the same type of move with identical function.
It is uncommon for clones to be starter characters; they are more likely to be unlockable characters. If a clone's parent is also unlockable, it is very likely for the clone to be unlocked after their parent.
The term "clone" does not by itself sufficiently describe how similar two characters are. Perhaps two characters share all but one attack, while another pair has only half of their attacks in common while still clearly being a derivative overall. As a result, there is a spectrum of terms to describe how much of a clone a character might be.
It is not uncommon for a cloned character to receive further differentiating changes in later games, thus making them less of a clone. Some use the term Luigification for when a character is separated from their parent and forms their own identity, as Luigi has done in both his home series and the Smash Bros. series. However, it is generally very difficult to determine at what point a character who was once a clone is no longer a clone, or even which specific category some characters may fall into. For example, Luigi’s status as any type of clone post-Brawl was a subject of heavy debate before the term “pseudo-clone” was accepted.
Ultimate introduces the term Echo Fighter to refer to certain clones. The term is used to determine which characters are grouped together on the character selection screen when the corresponding menu option is enabled, and also forces such characters to share the same fighter numbers with an appended epsilon (ε). Otherwise, it is simply a term used by the developers to denote a character as being low-budget; as a result, it only loosely fits into the spectrum of clone terms used by the community to discuss the degree of similarity in the final result. For example, Dr. Mario is a full clone, while Ken is a semi-clone, but Ken is labeled as an Echo Fighter while Dr. Mario is not. However, Daisy, Richter, and Dark Samus, who have next to no discernible differences from their base fighter, are labeled as such.
The Japanese term for "Echo Fighter" is Dash Fighter (ダッシュファイター), and uses the prime symbol (') instead of an epsilon. In Japanese, the prime symbol is pronounced "dash," hence the term "Dash Fighter"; in mathematics, the prime symbol is used to designate that something is derived from something else, such as
Clones in Super Smash Bros.
While all four of the unlockable characters were created with the premise of reusing some of the pre-existing characters' movements and models, two of them (Captain Falcon and Ness) are unique enough that they are not clones of their parents (Samus and Mario respectively), and mainly only copy from their skeletons rather than their movesets.
Clones in Super Smash Bros. Melee
Originally, Masahiro Sakurai was going to include significantly fewer playable characters in Melee. However, with fans wanting more characters while time was constrictive, Sakurai decided to add clones later in development to pad out the roster, as these characters would take a lot less time to develop than unique characters. According to the Japanese website, clones were known as model swap characters (モデル替えキャラ). Clones are marked on the character selection screen as recessed icons next to the fighters they are based on.
Luigi received many changes that distanced him from Mario considerably, changing him from a full clone into a semi-clone. On the other hand, Jigglypuff and Kirby both received changes significant enough that Jigglypuff is no longer considered any sort of clone.
Clones in Super Smash Bros. Brawl
None of the newcomers in Brawl are full clones, though there are new lesser clones. In addition, most of the clones from Melee were cut, with the two that returned (Falco and Ganondorf) now becoming semi-clones. As a result, there are no full clones in Brawl.
Clones in Super Smash Bros. 4
Super Smash Bros. 4 re-introduces full clones, adding one such veteran and two such newcomers. These clones were originally developed as alternate costumes, but it was later decided to give them a difference in moveset, resulting in them being split into their own characters — as "even a small difference in abilities" requires a unique roster slot. There are no new semi-clones, and returning semi-clones were not given much in the way of new differences. Clones are marked on the character selection screen by being listed outside of the rest of the characters from their series.
Most clones and semi-clones share at least one custom move, though full clones have more custom moves in common.
Clones in Super Smash Bros. Ultimate
As stated above, Ultimate uses the term "Echo Fighter" to refer to certain clones based on development time. The term is not fully related to how unique a character is, as some full clones are not marked as Echo Fighters despite their similar movesets. On the character selection screen, Echo Fighters are positioned directly after their base fighter, and there exists and option to merge the portraits of the base fighter and the Echo Fighter in certain modes.
With Ultimate bringing back all veterans, every clone that was originally cut returns. However, most returning full clones did not receive many new differences. In the transition from SSB4 to Ultimate, however, Luigi and Ganondorf were both significantly decloned (with Luigi commonly being agreed to have become a pseudo-clone) and Link's new changes based off of The Legend of Zelda: Breath of the Wild subsequently decloned both of his counterparts to an extent.