Shadow Mario is a character that has only appeared in the game Super Mario Sunshine. He is a disguise that Bowser Jr. wears to get Mario blamed his wrong doings. In the game, Shadow Mario vandalized the whole island of Isle Delfino, painting goop everywhere. When the real Mario showed up for his vacation, the Piantas saw him and thought he did it, because he looked like Shadow Mario. Mario was taken to court and told to clean up all of the goop on the island, even though he didn't do it.
Sometimes, Mario has to chase Shadow Mario, and spray him with his weak point, water, with F.L.U.D.D.. Most of the time, Mario must do this in Delfino Plaza when he kidnaps Princess Peach, and steals a Yoshi Egg. Occasionally when Mario enters a secret area, Shadow Mario will appear and snatch F.L.U.D.D. off of Mario's back.
Shadow Mario is also a playable character in Mario Golf: Toadstool Tour.