This article is from the 2006 series of SMBZ and is no longer canon.
Bowser Koopa Junior is Bowser's eighth son and is the youngest of the Koopalings after his big brother, Larry. However, he is his father's favorite child. Bowser Jr. is a perfect son to his father, as he wants to do nothing but his father's will. The evil king himself even shows emotions toward his new son, a side of Bowser that is rarely seen. Since he made his first appearance, he was working with his father years ago, causing trouble to Mario, Luigi, Yoshi and kidnaping Princess Peach. Meanwhile, his abilities have improved a lot recently, gaining some new incredible abilities and has now learnt how to spit fireballs in New Super Mario Bros. Wii. (but not at the same level as Bowser and Ludwig Von Koopa) He also uses his own powerful weapons and vehicles to fight against Mario (he now has his own personal Koopa Clown Car).
Because of him being the most perfect child for Bowser and the one who has caused trouble to Mario several times without giving up, his siblings' ranks are sightly lower than their father's Junior making the seven others technically subordinate to him. However, he is known to help them too. In New Super Mario Bros. Wii, Larry was the first one seen communicating with Bowser Jr.