Luigi (ルイージ, Ruīji) is the world famous plumber (the green one) and Mario's twin brother. Like Mario, his name comes from a common Italian masculine name (like Takeshi in Japanese). The theory that his name is similar to Mario's has been circulating, but it is a myth.
Initially, he was simply a different color of Mario, but in the animated film The Great Mission to Rescue Princess Peach! the appearance distinction was made, with a taller and slightly more slender design than Mario's. Later, this difference was incorporated into the games, and the mustache was changed to a less jagged one.
Japanese
ルイージ - アニヲタWiki(仮) - atwiki（アットウィキ） (Aniwota wiki - atwiki)
ルイージ/マリオパーティ9 | NeoApo アニメ･ゲームDBサイト (NeoApo Anime & Game Database Site)
ルイージとは [単語記事] - ニコニコ大百科 (NicoNico Encyclopedia)
