Kid Buu's History
Background
Majin Buu is said to be a creation of Bibidi by some sources. However it has been stated by Akira Toriyama that in truth Buu has actually existed since Time Immemorial. He cycled between rampages and long hibernation. During numerous iterations of this cycle, he absorbed the evil elements of mankind, becoming steadily more violent. The evil wizard Bibidi merely knew the means of calling Buu from out of his long slumber.
Long before the core events of Dragon Ball takes place, Majin Buu was summoned by Bibidi. This monster was a virtually invincible force of destruction, and, because of his absolutely wild and ruthless nature, he could not be controlled, even by Bibidi himself. An example of this is when Kid Buu blew up Planet Alpha and its inhabitants, and almost hit Bibidi in the process, causing Bibidi to tell Kid Buu off and only meekly telling him he is his "father" when Kid Buu intimidated him by glaring at him. Eventually, Buu and Bibidi made their way to the Sacred World of the Kai, where the rulers of the universe and Bibidi's sworn enemies, the Supreme Kais, lived peacefully. Majin Buu proceeded to kill both West Supreme Kai and North Supreme Kai before encountering South Supreme Kai, the strongest of the Kais. Although the Kai managed to fight more evenly with Buu than his cohorts had and landed a few hits on the monster, Buu managed to absorb Southern Supreme Kai during their fight, which transformed him into a bulkier version of himself.
Majin Buu then made an attempt on the life of Shin, but he was saved by Grand Supreme Kai, who inflicted immense damage on Buu, but was unable to kill him due to Buu's regeneration. Buu then absorbed him, resulting in the childlike fat Majin Buu. Due to the innocence of the Grand Supreme Kai's soul, this Buu was much less evil and wild than the original Buu. As Buu became calmer and easier to control, seeing his opportunity, Bibidi finally managed to seal Majin Buu inside of a Sealed Ball without much resistance from the latter. Bibidi was then killed by Shin a short time later, which meant the madness of Majin Buu was believed to have ended for good.
However, the fat Majin Buu was again resurrected 5 million years later on Earth by Bibidi's doppelganger, Babidi. After a long chain of events (which includes the fat Majin Buu murdering Babidi, the fight between Good Buu and Evil Buu, the birth of Super Buu, and everyone that Super Buu had absorbed being removed by Goku and Vegeta), the original pure evil form of Majin Buu finally resurfaced.
Due to the destruction caused by Kid Buu, Majin Buu developed a fearsome reputation in Universe 7, enough that King Cold warned Frieza to never oppose Majin Buu.
Majin Buu Saga
After he came to his senses, transforming from Super Buu after Vegeta removed the good Majin Buu from Super Buu's head, Kid Buu attempted to destroy the Earth with a small yet incredibly powerful energy ball, but it was deflected by Vegeta's Final Burst Cannon. Kid Buu then created a massive energy ball and flung it towards the Earth. Goku and Vegeta realized that they would be unable to stop it and made an attempt to escape with Gohan, Goten, Trunks, and Piccolo. However, the only three that they were able to grab nearby were Mr. Satan, his dog Bee, and Dende. Kibito Kai appeared and grabbed onto Goku's hand (and grabbing on to Vegeta by his sleeveless jumpsuit), and teleports them away with Kai Kai while Gohan, Goten, Trunks, Piccolo, Yajirobe, Korin, Captain Ginyu, Tien Shinhan, and Chiaotzu are left on Earth, where it is then shortly and completely obliterated.
Regenerating after the explosion, Kid Buu goes off in search of Goku and Vegeta, teleporting to many planets as he goes and destroying them upon sensing the Saiyans' absence. Along the way, in a filler scene, he finds the Grand Kai's Planet, where Pikkon and Olibu approach him and start taunting him, leading a number of dead warriors risking their existence, against him including Krillin and Yamcha. After a victorious scuffle with Krillin, before he can finish him and the other dead warriors, Kid Buu manages to sense the increasing combat powers of Goku and Vegeta, and teleports off to the Sacred World of the Kais seeking strong opponents. Upon arriving, Kid Buu battles with Super Saiyan 2 Goku and Goku has the upper hand, although Kid Buu is not fighting at his maximum. Once he powers up, Goku transforms into Super Saiyan 3 and fights evenly with him.
Kid Buu, having the advantage of perpetual endurance, wins over the battered Goku, who is quickly losing power from his draining Super Saiyan 3 transformation. Vegeta transforms into his Super Saiyan 2 form to fight Kid Buu in an attempt to buy some time so Goku can charge to full power to finish Buu, but is quickly defeated as well despite fighting well and lasting longer than expected. Mr. Satan then steps in and attempts to fight Buu, but is severely outmatched and relies on mere luck to survive. Kid Buu moves in for the kill, but finds himself unable to do so due to having the memories of Good Buu. His frustration ultimately leads him to spit out a small stone which explodes into an unconscious Good Buu. Now with nothing to stop him, Kid Buu resumes the play-fight with the enraged Mr. Satan, who attempts another pitiful attack when Good Buu steps in. Good Buu angrily tells Kid Buu to leave Mr. Satan alone and that his battle is with him. Kid Buu and Good Buu then engage one another in combat. The battle quickly turns in Kid Buu's favor, and he ultimately dominates Good Buu, who however, refuses to give up the fight.
Meanwhile, Goku and Vegeta hatch a plan to have Dende and Kibito Kai use Porunga to restore the Earth and revive all the good people who were killed since the World Martial Arts Tournament. Goku then prepares to create a Super Spirit Bomb, while Vegeta uses King Kai's telepathy to ask for the Earthlings' help, to which only their friends and family respond. Kid Buu and Good Buu continue their battle, until Good Buu is beaten so badly that he cannot fight any longer. Mr. Satan then steps in to stop Kid Buu, and Vegeta once again battles Kid Buu to buy some more time. Ultimately, Mr. Satan manages to convince the people of Earth to give up their energy and the Super Spirit Bomb reaches completion, frightening Kid Buu. Unfortunately, Kid Buu holds the battered Vegeta 'hostage', keeping him in the Spirit Bomb's path as well, therefore stopping Goku from throwing the attack. However, Good Buu wakes up and smashes himself headfirst into Kid Buu, knocking him off Vegeta for Mr. Satan to carry off to safety. Unable to get Good Buu off of him, Kid Buu angrily blasts him away with a massive energy sphere.
Now with an obstacle-free path, Goku takes his chance and throws the Spirit Bomb straight at Kid Buu, but Kid Buu is able to stop it and slowly pushes it back towards the exhausted Goku. Dende then uses the third wish with the Namekian Dragon to have all of Goku's strength be restored. Goku then turns Super Saiyan and overpowers Kid Buu with the Spirit Bomb, causing Majin Buu to be obliterated and thus vanquished forever. Shortly before Kid Buu's destruction, Goku openly hopes that one day Kid Buu will return as a good person so that they may have a rematch. In the Other World, King Yemma hears Goku's request and makes it come true by having Kid Buu reincarnated as a Earthling named Uub. In order to settle things for good with Good Buu, Goku wishes to erase all the Earthlings' memories of Majin Buu. Mr. Satan also gives Good Buu strict orders to refrain from getting angry, no matter what.
