Wario

Wario is an anti-hero in the Mario franchise, who eventually became popular enough to spawn his own titular franchise. Wario first made a cameo appearance in Super Smash Bros. Melee before debuting as a playable character in Super Smash Bros. Brawl.

Super Smash Bros.
Wario does not appear in the original Smash game. However, a rumour states that Wario was planned to be playable in the game before being cut due to time constraints. Despite this, no source has ever been found that proves that Wario was ever meant to be playable.

Instead of being a playable character, one of Mario's alternate costumes resembles the colour scheme of Wario.

Super Smash Bros. Melee
Wario was a highly requested character for Super Smash Bros. Melee's roster, and Masahiro Sakurai stated that he could have added Wario if he had time to add one more character. Sakurai was also concerned that the game would have too many Mario characters. Sakurai also stated that he could have used development time dedicated to either Mewtwo or Marth and Roy to add Wario instead, but he decided against this.

Wario is once again represented in the game in one of Mario's alternate costumes, and Wario is also an unlockable trophy. To unlock the trophy, the player has to beat All-Star Mode without a continue.

Super Smash Bros. Brawl
Wario makes his first playable appearance in Super Smash Bros. Brawl, and is one of the starter characters. His main design draws inspiration from the WarioWare series. Wario is also a very heavy character, which makes him hard to KO.

Super Smash Bros. for Nintendo 3DS / Wii U
Wario returns as a fighter in Super Smash Bros. for Nintendo 3DS / Wii U. He is now an unlockable character instead of a starter character. He can be unlocked by beating 100-Man Smash or playing 20 VS Matches on the Wii U. For the Nintendo 3DS, the normal unlock criteria is the same, but the player has to take part in 30 VS Matches to unlock Wario.

Super Smash Bros. Ultimate
In Super Smash Bros. Ultimate Wario returns as a playable fighter along with all the other fights. There are three different ways to unlock Wario: through World of Light, playing 3 hours and 50 minutes of VS Matches and having 23 other characters unlocked, or clearing Classic Mode seven times as Samus Aran or anyone in her unlock tree. Clearing one of these tasks triggers a fight against Wario in a battle, and once Wario is defeated, he becomes a playable character.

Trivia

 * According to Masahiro Sakurai, he stated that Dr. Mario was ultimately included in Melee over Wario due to time constraints. Whether this is true or false is still not clear.