Bowser Jr., known in Japan as Koopa Jr. (not to be confused with Baby Bowser in Super Mario World 2), is the eighth and youngest child of Bowser that debuted in Super Mario Sunshine. He acts as the main antagonist of both Super Mario Sunshine (where he first appears as Shadow Mario) and New Super Mario Bros., with his father only playing a minor role in each. He also appears in the various sports games, often as a "Speedy" or "Tricky" character. He also appears in Super Mario Galaxy as a boss. He appears in Super Mario Sluggers as the enemy of story mode and a playable character. Bowser Jr. shares many physical similarities with his father, with the main differences being a red ponytail as opposed to Bowser's mane, small black beady eyes, a more rounded snout, and a single sharp tooth visible in his mouth--features which, along with his body shape, make him look almost identical to Bowser himself at a younger age. He wears a kerchief constantly. It was originally a drawing of Mario's distinctive nose and mustache, but it is later replaced with a set of sharp teeth.
In Super Mario Sunshine, Bowser Jr. uses a magic paintbrush, invented by Professor E. Gadd, and a special kerchief to transform into a translucent blue doppelgänger of Mario, Shadow Mario, who vandalizes Isle Delfino, tricking the native people into believing that Mario himself is to blame. He does this to Mario because Bowser tells him that Princess Peach is his mother, and that Mario has kidnapped her. Bowser Jr. kidnaps her himself, and is eventually defeated along with Bowser. He realized that Princess Peach isn't his mother before Bowser decided to tell him, but he still refers to Peach as "Mama" sometimes in later games, such as when he challenges Princess Peach to a game in Mario Superstar Baseball. In Mario Strikers Charged he is the champion of the Fire Cup. After beating the cup players will unlock Bowser Jr. as a playable character. He is good at shooting and passing and bad at everything else. His special is Sonic Scream, which turns all affected characters smaller.