Wario

 is an anti-hero in the Mario series, who eventually became popular enough to spawn his own franchise of video games. He made a cameo appearance in Melee before debuting in Brawl as a playable character.

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 Melee's roster, and Masahiro Sakurai stated that he could have added Wario if he had time to add one more character in Melee. 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 Brawl. 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 in SSB4 as a fighter. 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
Wario makes an appearance in Ultimate as a playable character. He returns with all other characters.

Wario can be unlocked in one of 3 different ways. He can be unlocked by playing 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 or anyone in her unlock tree. Beating one of those tasks will result in Wario going up against the player and challenging them to a battle. If the player is successful, Wario will be playable.

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 not is still not clear.