Bowser Jr. is the mischievous son of Bowser and the prince of the Koopas. He resembles his father's baby form, having beady eyes, a rounded snout with fewer contours, and hair tied in a small ponytail. He debuted in Super Mario Sunshine, where he painted over most of Isle Delfino with a magic paintbrush he received (or stole; this is never clarified upon) from Professor E. Gadd. The "paint" used was actually goop, which was very toxic, so much so that when spread over the Shine Gate, the sunlight over Isle Delfino depleted. Bowser Jr. committed his crimes disguised as Mario, tricking the denizens of Isle Delfino (known as Piantas) into thinking Mario was the culprit. These crimes were committed with the main intent of capturing Princess Peach, believing she is his mother. In his disguise, Bowser Jr. was similar to Mario but had a higher-pitched voice and a gelatinous, dark blue body with glowing red eyes. The disguise was activated by a blue neckerchief with Mario's signature mustache drawn on it (in later games starting with Mario Golf: Toadstool Tour, Bowser Jr.'s neckerchief is white and has a drawing of a ferocious mouth instead). Bowser Jr. was also convinced by his father that Peach was his mother and that Mario had taken her from him, so he attempted numerous times to kidnap her (as well as always referring to her as "mama"); however, by the end of Super Mario Sunshine, Bowser Jr. already knew that Peach wasn't really his mother, but he still expressed his father's desire to fight Mario.
Later Mario games would explore more facets of Bowser Jr.'s character. In Super Mario Galaxy and subsequent games, Jr. has been shown to pilot large battle machines of various sorts, and his mechanical battle angles show through in his Super Smash Bros. moveset. Mario & Luigi: Paper Jam explored Jr.'s personality in further detail, showing that, while he may be a spoiled child and a cunning villain, he is ultimately a lonely Koopa who desires friends above anything else.
Bowser Jr. became a landmark character after Super Mario Sunshine, appearing as a playable character in multiple spin-off games (his first being Mario Golf: Toadstool Tour) and having a recurring role as Bowser's elite soldier in the New Super Mario Bros. games. There, he uses a smaller variant of his father's famous Koopa Clown Car, officially named the Junior Clown Car in Mario & Luigi: Dream Team. Most notably, he appears in Mario & Luigi: Bowser's Inside Story + Bowser Jr's Journey, the 3DS remake of Mario & Luigi: Bowser's Inside Story, and takes center stage as the main protagonist of the untold story in Bowser Jr's Journey.
Bowser Jr. appears as a trophy and as two stickers.
As a playable character
Main article: Bowser Jr. (SSB4)
Bowser Jr. was unofficially revealed as a playable character in SSB4 by a leak of the ESRB review video. He was shown to fight in his Junior Clown Car, using boxing gloves to attack (inspired by his Clown Car in New Super Mario Bros. U, but the gloves protrude with metal extenders instead of being attached to arms). He can also drop Mechakoopas and fire cannonballs. The seven Koopalings also appear as alternate costumes for the character. His Final Smash has him briefly transform into his Shadow Mario form, which then swipes the screen with the Magic Brush, creating a large X, which hurts all opponents that it touches.
Currently, Bowser Jr. is placed 48th out of 55 on the tier list, placing him in the F tier. His Clown Kart Dash special gives him a fantastic approaching option thanks to its speed and usefulness in starting combos. He also comes with some powerful finishers that can potentially KO opponents below 100%. Although he's classed as a heavyweight, his aerial game is respectable thanks to his good air mobility and ability to juggle opponents with his aerials. Despite these strengths, Bowser Jr. is held back by notable flaws, such as a laggy grab, poor ground mobility, and his recovery in Abandon Ship! is predictable and open to gimps. Bowser Jr.'s weaknesses hold him back from leaving much impact on the competitive scene, having below average results in tournament, though he still has a few dedicated players.
As a playable character
Main article: Bowser Jr. (SSBU)
Bowser Jr. returns as a playable fighter in Super Smash Bros. Ultimate. Like in Super Smash Bros. for Nintendo 3DS, Bowser Jr. is an unlockable character.