Super Mario Sunshine was Bowser Jr.'s first appearance in the Mario franchise, where he was tricked by his father, Bowser, into thinking that Princess Peach was his mother. Prior to the events of the game, he managed to get a magic paintbrush that allows him to create a goop-like substance.
He is able to take the form of Shadow Mario and uses this ability to try and frame Mario by spreading this goop substance all over Delfino Isle. He also uses this disguise to cause all sorts of trouble and mayhem for Mario. In the end, he is defeated by Mario, along with his father.
