Bowser Jr. is the youngest and eighth child of Bowser. Bowser Jr. is the only one of Bowser's children who is not part of the Koopalings.
Super Mario Sunshine
Bowser Jr. first appeared in Super Mario Sunshine as the main antagonist, who disguised himself as Mario, using a magic paint brush and bandanna, invented by Professor E. Gadd and then vandalized Isle Delfino. This caused the people of the island to turn against Mario, who was vacationing on Isle Delfino.
Following in the footsteps of his father, he kidnaps Princess Peach after Bowser tells him that Peach is his mother. Bowser Jr. then takes Princess Peach to Corona Mountain where he plans on going on his own vacation with his newly found mother.
However, Bowser Jr. and his father are both eventually defeated by Mario and Princess Peach is saved yet again.
New Super Mario Bros. Series
New Super Mario Bros.
Bowser Jr. takes a much more direct approach in New Super Mario Bros., simply kidnapping Princess Peach upright. Bowser Jr. once again was teamed up with his father but once again, they were both defeated by Mario. This also marks the first time Mario had actually killed Bowser, though Bowser Jr. was able to resurrect his father twice, once in a skeletal form and then again in his normal form.
New Super Mario Bros. Wii
In addition to the DS debut of New Super Mario Bros., he also appears in the Wii sequel where gets to kidnap the princess again with the Koopalings but at the end of the game, he gets crushed by his dad's castle.
Bowser Jr. also appears in Super Mario Galaxy at the right hand of Bowser, fighting Mario at various times throughout the game. He will make a return in Super Mario Galaxy 2.
Bowser Jr. has also appeared in every Mario sports game since he was introduced in Super Mario Sunshine.
In Mario Party DS, he made his debut in the Rocket Rascals sub-game where he likes to mess the game around.
In Mario & Sonic at the Olympic Winter Games, he is an All-Around character and for the first time ever in the Mario & Sonic series, he is playable.
Bowser Junior is Bowser Koopa's son