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From Unmariowiki

Luigi is a character who first appeared in the 1880s as one of the Super Mario Cookies. These cookies came a years after so many fans loved the Mario Cookies. The Luigi Cookies were simply a recolor of the Mario ones.

Later he appeared in the Mario videogames, as Mario's long lost twin. They met in a crash-both of their cars crashed ad after they saw how similar they were, they realized they were twins. Luigi was Mario's sidekick for the first game but was then kicked out of the series.

Until in the Videogame War, when Nintendo didn't want to make Mario games, and instead started creating Luigi games. Luigi got his girlfriend (Daisy) his own series, his own games his own enemy (Queen BOO!

After the war stopped, Luigi was forgotten again. Luigi's latest apperance is in a now cancelled TV show called George W. Bush's Idiot Adventures.