Bowser Jr. is the last son of Bowser (not to be confused with Baby Bowser) and the secondary antagonist of the Mario franchise. He looks just like Bowser except younger and he constantly wears a bib with fangs drawn on it perhaps to look more intimidating. He seems to be the direct superior to the Koopalings, even though they are his older siblings.
Appearances
His first appearance was in Super Mario Sunshine where he tried to frame Mario for crimes he didn't commit. This plot actually succeeded. He took (presumably stole) a magic brush from Professor E. Gadd and used it to turn into Shadow Mario (for framing Mario) and make evil graffiti appear. Mario was instructed to clean up the graffiti, and soon defeated Bowser Jr. inside a giant Bowser machine, at which point Bowser Jr. revealed his true identity and that Bowser told him that Peach was his mother. After Mario chased him out of every world, he fought Bowser in a giant floating hot tub above Corona Mountain. Bowser Jr. sent Bullet Bills after him, but he and Bowser were defeated, and last seen in the middle of the ocean where Bowser told Bowser Jr. the truth about Peach, and Bowser Jr. said he wants to fight Mario again.
Bowser Jr. playing ice hockey with his father in Super Mario sports mix.
He has been seen in other Mario games since then. He even appeared as a final boss along with his dad in the New Super Mario Bros. game for the DS. He is the main antagonist of this game; it is he who kidnaps Peach for Bowser, and summons a boss to each level for Mario and Luigi to fight. He is fought in a tower at each level. When Mario kills Bowser by knocking him into the lava, Bowser Jr. takes the bones and eventually brings Bowser back to life (it is unknown how many other times throughout the series he has needed to do this, if any). Bowser and Bowser Jr. fight Mario, but both are defeated and knocked over the edge. Both survive the fall, and Bowser Jr. is last seen dragging Bowser away by the tail.
Bowser Jr. also appeared as a boss fought several times in Super Mario Galaxy. The first time he sends a giant robot named Megaleg after Mario, but Mario destroys it. The second time Mario fights him and his airship, and the third time he merely sends King Kaliente after him. He is also fought three times in New Super Mario Bros Wii. (the only game he appeared in alongside his brothers and sister, the Koopalings). The first time he can be jumped on with propeller blades, the second time he must be bumped into electric walls, and the third Mario must stomp the ground to hit a bomb back at him. He survived his third defeat (Kamek rescued him from the previous two), and was seen alongside the Koopalings helping Bowser back up, when Bowser's castle fell over and crushed them.
Aside from mainstream games, he has also appeared in spinoffs such as Mario Kart: Double Dash, as a playable character.
Shadow Mario
The Aspect used by Bowser Jr. to incriminate Mario, using a magic paintbrush acquired (presumably stolen) from Professor E. Gadd. He made graffiti all over Delphino Island and framed Mario for such vandalism, making the inhabitants of the island punish Mario by having him clean up all the mess Shadow Mario had created, under the belief that it was Mario who was the real villain.
However during his cleaning up of the island Mario soon comes into contact with Shadow Mario, who tries to kidnap Princess Peach and the two would fight on and off throughout the game.
Super Smash Bros. 4
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Bowser Jr. in Super Smash Bros 4
It's been confirmed that Bowser Jr. will appear as a playable character in the video game; Super Smash Bros 4 for the WiiU and 3DS. When played, Bower Jr. will use his Junior Clown Car for combat and his Final Smash is going to be Shadow Mario. It's also been confirmed that all seven of his alternate skins will be the Koopalings riding the Junior Clown Car; Larry Koopa, Morton Koopa Jr., Wendy O. Koopa, Iggy Koopa, Roy Koopa, Lemmy Koopa, and Ludwig von Koopa.