Luigi (ルイージ, Ruīji) is a character in the Mario series. He is the younger brother of the more-famous Mario, but is still a prominent character (and second only to his older brother) in the series of the same name. He has featured as a playable character in Super Smash Bros., Super Smash Bros. Melee, and Super Smash Bros. Brawl, and will be a playable character in Super Smash Bros. 4.
Interestingly, Luigi is only one of two characters (alongside Jigglypuff) to appear in all three Super Smash Bros. games as an unlockable character. The criteria to unlock him often has a relation to the number 2, in reference to his status relative to Mario.
 Character description
Two years after his brother, Mario, made his first appearance in the Arcade classic Donkey Kong, Luigi would make his first appearance in Mario Bros for the arcade as the character the second player would control. His appearance is a palette swap of Mario, the difference being he's colored with green instead of red. He was named after a pizza parlor which was near Nintendo of America's headquarters called "Mario & Luigi's".
Luigi would then once again be featured in the world-famous Super Mario Bros. for the NES, which saw the two brothers traveling through the Mushroom Kingdom for the first time to rescue Princess Peach (then called "Princess Toadstool") from the evil Koopa King, Bowser. Here Luigi was clad in a green shirt, white overalls and white hat contrasted to his usual green color scheme. In Super Mario Bros. 2 (both the U.S. and Japanese versions) he would be established as actually having a superior jumping ability to Mario, but having worse traction. However, the U.S. Version did establish that Luigi was both taller and thinner than his older brother. In Super Mario Bros. 3, Luigi once again looked no different than Mario.
Over the years, while Mario appeared in more and more games, Luigi started taking a back seat to his older brother, even being left out of some games entirely. While he appeared as the solo playable character in Mario is Missing!, this was not a "true" Mario title. The most notable of these was Luigi's complete absence in the world-famous Super Mario 64 which garnered some controversy. He also did not appear in Super Mario Sunshine after that, although he did appear in the Nintendo DS remake of Super Mario 64 as a playable character along with Yoshi and Wario.
However, he would soon make his first appearance in a fighting game in Super Smash Bros. and following that, garnered the spotlight in his own game Luigi's Mansion which launched the GameCube. Since then, Nintendo has made a more conscious effort to include Luigi in more games, playable or otherwise, he starred alongside Mario in the Mario and Luigi games for the Game Boy Advance and Nintendo DS and was a playable character in Super Paper Mario (after appearing as a non-playable character in the first two Paper Mario games). He has even been made into a playable character in Super Mario Galaxy after the game is beaten as Mario. In Super Mario Galaxy 2, he makes a playable appearance by temporarily replacing Mario for the duration of certain levels. Things have since been looking up for Luigi, and while he is less famous than Mario, he is a popular character among gamers.
 In Super Smash Bros.
 As a playable character
Luigi's in-game character description reads:
 In Super Smash Bros. Melee
 As a playable character
Luigi returns in Melee as a secret character. He can be unlocked in the adventure game by finishing the first stage with the number "two" in the timer, which will cause Luigi to take Mario's place in the subsequent battle, or by playing 800 VS. matches. Finishing the first stage with the number "two" in the timer may be a reference to the fact that Luigi has always been considered second to Mario. Luigi's side special move gives him a unique ability, the Green Missile. Luigi charges up, and when this is finished, launches himself forward as though he were a missile (hence the title). This can be used as a recovery move along with his Super Jump Punch. He also has many all-new Normal Moves. Luigi is ranked 13th on the tier list in A tier, placing him in the very middle along with his brother Mario.
As a playable character, Luigi is featured on three trophies - his normal trophy, acquired by beating Classic mode with Luigi on any difficulty, and "Smash Red" and "Smash Blue" trophies won by beating Adventure and All-Star modes, respectively - as well as a fourth, styled after his appearance in the GameCube launch title Luigi's Mansion".
His normal and "Vacuum Luigi" trophies read as follows:
 In Super Smash Bros. Brawl
Luigi is once again an unlockable character and can be unlocked by playing twenty-two vs matches, completing Classic Mode, or by getting Luigi to join the party in Adventure Mode: The Subspace Emissary. The main difference between Luigi now and his Melee appearance is his Final Smash, the unorthodox Negative Zone. Luigi is ranked 28th on the tier list in E tier.
Mario's younger twin brother. He's shy and quiet and overshadowed by his sibling, but he's actually quite talented. His jumping ability surpasses Mario's, and his all-around skills let him overcome any problem. He's a bit cowardly and really afraid of ghosts. Even so, in Luigi's Mansion, he was charged with cleaning up a whole house full of spirits.
 In Super Smash Bros. 4
Luigi has been confirmed to appear in Super Smash Bros. 4.