300 years ago an evil magician named Bibidi created the Majin Buu, a demon who's intent is pure evil and destructive . Bibidi could not fully control Buu at first. Eventually Buu killed all but one of the Kaioshins . The east and west Kaio-shins were easily defeated. The south Kaio-shin put up a good fight but was absorbed by Buu at the last minute. This caused Buu to inherit the physical traits of the south Kaio-shin.

Buu then absorbed the master Kaio-shin and transformed into the fat Buu . Since master Kaio-shin was kind, gentle and fat, Buu was watered down a bit and easier to control. Majin Buu was later resurrected by his son Babidi. Buu goes thru many transformations including returning to his first state. The remaining Kaioshin sealed and sent the demon towards a certain random planet and was forgotten for 300 years.