Bowser Jr.

"I know, she's not really my Mama."

- Bowser Jr. after learning that Princess Peach is not his mother.

"Someday... when I'm bigger... I wanna fight that Mario again!"

- Bowser Jr. to his father.

Bowser Jr. is the only biological son of Bowser and one of the eight children in his care (not to be confused with Baby Bowser).

Since his debut in Super Mario Sunshine, he is the secondary antagonist of the Super 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 older than him.

History
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.

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.

In the game, Mario & Luigi: Paper Jam, Bowser Jr. meets his paper self and unlike, his father, he got along with him. As Mario, Luigi, and Paper Mario defeated both of them in a boss battle, both Bowsers come in to comfort them after they defeat. As Both Bowser Jr's lay down in their arms, they both try to tell their fathers about the book, but their fathers hushes them as they both said they're delirious.

Aside from mainstream games, he has also appeared in spinoffs such as Mario Kart: Double Dash, as a playable character.

Shadow Mario
In Super Mario Sunshine, 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
Bowser Jr. made his debut as a playable character in the video game; Super Smash Bros 4 for the WiiU and 3DS. He is an unlockable character in the 3DS version, but he is available from the start in the Wii U version. When played, Bowser Jr. uses his Junior Clown Car for combat. The Junior Clown Car contains a large abundance of weapons include; shooting a cannonball out of the mouth, lashing the opponent with it's tongue, retractable robotic arms that contain either drills, or boxing gloves, retractable wheels so Bowser Jr. can ram into his opponents, distribute Mechakoopas, drop two iron balls, and even explode if Bowser Jr. abandon ship. Bowser Jr. also carries a hammer to defense himself when he abandons the Junior Clown Car.

Bowser Jr.'s Final Smash is Shadow Mario. Bowser Jr. transforms into Shadow Mario, and uses the magic paintbrush to create an X-shape mark on the screen. Anyone who comes in contact with the mark will be dealt with damage. The effect lasts for a few seconds, when the mark explodes and launches everyone touching it. For his alternate costumes; all seven of them alternate are 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.

Physical appearance
Bowser Jr. is pretty young, he has a yellow skin like his father, he has small, circular eyes that are black in color. His mouth is wide, puffy and con-vexed, with a large snout, Bowser Jr. has little hands and little feet, which have small white claws, he wears a white bandanna with a drawing of a scary mouth, he also has a four-ribbed padded stomach and padded feet.

Personality
Bowser Jr. seems to be very bratty and spoiled. He also seems to be a little bit of a psychopath since he believed that Princess Peach is his mother, however, he is like this because he is raised by his awful father who pretty much made him like this. Despite that, they have a genuine father-son relationship and really care for each other, shown in games like Mario + Rabbids where Bowser calls Junior on the phone to make sure he did his homework.

Trivia

 * The pads of Bowser Jr's feet are similar to Birdo's.
 * Since his name is Bowser Jr., this would mean that his father's real name is Bowser Sr.
 * Bowser Jr. is hinted to be 4-5 years old.