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The initial from of Majin Buu from Dragon Ball Z, also known as "Good Buu" or "Mr. Buu". This form is a result of Buu absorbing the Dai Kaioushin and the good and evil energy of Earth during the millennia of his sealing, resulting in his schoolyard bully personality and fat appearance.
After Babidi revived Buu from his slumber, Buu went on to cause a rampant destruction on earth, mostly to satisfy his hunger by turning any lesser being into candy and chocolate. Mr. Satan later befriended Buu and taught him virtuous values, but soon after he witnessed his adopted stray dog being killed by a group of bounty hunters who were sent to kill him, Buu went berserk and killed the bounty hunters in the process. This would result in his evil alter-ego being unleashed, who overpowers him and absorbs him back in turn, creating Super Buu.
After Super Buu was forced to release Fat Buu along with the other victims he absorbed, Fat Buu would become a separate entity, while Super Buu would transform into Buu's pure evil form, known as Kid Buu.
This tag implicates majin_buu (learn more).