F.L.U.D.D. (mostly called FLUDD) first appeared as one of the main characters in Super Mario Sunshine. He aided Mario in his quest to save Princess Peach. He was invented by Professor Elvin Gadd (also called Egadd). Since then F.L.U.D.D. has appeared in numerous cameos.
Super Smash Bros. Brawl
In Brawl, F.L.U.D.D. will be Mario's new Down B move, replacing Mario Tornado.