A player's main character, often shortened to main, is the character a person uses the most often. For instance, Ken's main in Super Smash Bros. Melee is Marth. "Main" also can be used as a verb, e.g., "Isai mains Captain Falcon and Sheik". In contrast, secondary characters are used less often or in specific situations.
It is possible for a player to use more than one main character. Instead of having secondaries, some players use characters equivalently in conjunction with each other. In tournaments, multiple mains are more common with low tier mainers as they usually require more than one character in order to face difficult matchups. Players maining a single character solo main them, and players maining two characters dual main them.
One may also main different characters in different games, like how ZeRo mains Fox in Melee, Meta Knight in Super Smash Bros. Brawl, Diddy Kong in Super Smash Bros. 4 and (formerly) Wolf in Super Smash Bros. Ultimate. This is common, seeing as many characters' tournament viability can fluctuate in different games.
The sub-term character loyalist or character specialist refers to someone who mains specific characters throughout different Smash games with a degree of devotion, regardless of viability. Notable examples include Hungrybox and Larry Lurr who respectively use Jigglypuff and Fox/Falco in every game. Many times, a loyalist may not play a Smash game for the sole reason of their main being nerfed, unviable, or being absent entirely from a particular title in the series. Some players argue in favor of maining different characters in each Smash game, believing that character and gameplay alterations in one Smash title can carry over into a smasher's playstyle in another, negatively affecting their performance with the character in tournament.
The opposing terminology would be a random character main, as in someone who mains no one and uses the entire cast, though this is predominantly a non-competitive ideology. On the other hand, someone who can play the majority of the cast and can demonstrate competent tournament play with versatility; (such as Ryo), could play a random character in a serious setting even when they have a main (Ike in Ryo's case). Some jokingly say "maining the random button" when talking of such players.