Bowser Jr. is Bowser's only child, the Koopa King's heir to the throne, and the secondary antagonist in the Mario series. Bowser Jr. first appeared in Super Mario Sunshine, and has helped Bowser kidnap Princess Peach, and battle Mario and Luigi in many subsequent games. Bowser Jr. wants nothing more than to do his father's evil will, and Bowser's rarely seen emotional side is often brought out by his interactions with his son.
Bowser Jr. first appears in Super Mario Sunshine as the game's main antagonist. Before the events of the game, Bowser Jr. somehow acquires the Magic Paintbrush, a special brush that can magically create sticky paint-like Goop, from the scientist Professor E. Gadd. Bowser Jr. is also in possession of a bandanna emblazoned with a logo resembling Mario's nose and face that apparently lets him assume the appearance of a watery-blue Mario. Bowser Jr. uses this ability to assume the identity Shadow Mario. As Shadow Mario, Bowser Jr. paints magical Graffiti all over the tropical resort Isle Delfino, in an attempt to frame Mario for the crimes.
personality
Bowser Junior is a bit bratty and stubborn, not accepting defeat, always thinking of a reason why he lost. He is also very hyperactive and cocky.
abilities
Bowser Jr. has a diverse line of abilities himself, but his preferred combat method is either controlling a mech, or using his Shadow Mario form.
Also noted, Bowser Jr. wields a paint brush that spreads an infinite amount of goop, using this goop, he can create strange portals that lead to new areas, such as Bianco Hills in the picture above.