ABOUT KID BUU
Kid Buu is the original and purest form of Majin Buu; even though this form is the last one that he appears in. This Buu appeared after all of the fighters that he'd absorbed were removed from his system. Unlike his previous forms, this form of Buu's thought process is seen to be irrational and spontaneous, even destroying his own body to destroy the Earth.
Unlike his anime counterpart, in the manga series, this Buu is never seen to be capable of speech (similarly, the Evil Buu whose existence stemmed from Fat Majin Buu's anger is never shown to be speaking in the manga before his transformation into Super Buu). The first form he appears in is actually the form that he took after he'd absorbed Grand Supreme Kai millions of years earlier. When inside Super Buu, Goku and Vegeta rip Good Buu out, causing Super Buu to revert to this form. Although he appears smaller than any other form, he is more dangerous than any other forms of Buu, due to his full relentless evil and unpredictability. He is also completely free of restraint, holding his own against Super Saiyan 3 Goku, Good Buu, and Super Saiyan 2 Vegeta. In this form he has no trace of sanity, and because of this is extremely volatile. He retains his childish personality, becoming amused and laughing hysterically at his own destruction of entire worlds. In addition, he is the final villain of Dragon Ball Z.
Majin Buu was created roughly 5 millions of years ago (before the actual story of Dragon Ball takes place) by an evil wizard named Bibidi. This monster was a virtually invincible force of destruction, but because of his absolutely wild and ruthless nature he could not be controlled even by Bibidi himself. Eventually, Buu and Bibidi made their way to the Planet of the Kais, where the rulers of the universe and Bibidi's sworn enemies, the Kaioshin, lived peacefully. Majin Buu proceeded into killing both the Western Supreme Kai and the Northern Supreme Kai before encountering the Southern Supreme Kai, the strongest of the Kais. Buu managed to absorb Southern Supreme Kai during their fight, which transformed him into a bulkier, much stronger version of himself (dubbed as Ultra or Mega Buu by many fans).
Majin Buu then made an attempt on the Eastern Supreme Kai's life, but is saved when the Grand Supreme Kai came into the scene. After a brief struggle, Buu absorbs Grand Supreme Kai which results in the childlike Fat Buu who was much less evil than the original Buu due to the innocence of the Grand Supreme Kai's soul. Seeing his opportunity, Bibidi finally managed to seal Majin Buu inside of a magic cocoon without much resistance from the latter. Bibidi was then killed by the Eastern Supreme Kai 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 son, Babidi. After many chains of events (which includes the Fat Buu murdering Babidi, the fight between the Good Buu and the Evil Buu, and the birth of Super Buu), the original pure evil Kid Buu finally resurfaces, after everyone inside of Super Buu (who absorbed Gotenks, Piccolo, and Gohan) had been removed by Goku and Vegeta (who were voluntarily absorbed when they were Vegito).
After he comes to his senses, Kid Buu attempts to destroy the Earth with a small yet incredibly powerful energy ball, but it is deflected by Vegeta. Kid Buu then creates a massive energy ball and flings it toward the Earth. Goku and Vegeta realize that they would be unable to stop it and make an attempt to escape with Gohan, Goten, Piccolo, and Trunks. However, the only two that were able to grab nearby was Mr. Satan, Bee, and Dende. Kibito Kai appears, grabs onto Goku and Vegeta, and teleports away with Instantaneous Movement.
Kid Buu as he prepares to blow up the Earth.
Gohan, Goten, Piccolo, and Trunks are left on Earth when it is 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. Eventually, Kid Buu manages to sense the increasing combat powers of Goku and Vegeta, and teleports off to the Kai Planet for his desired rematch. Upon arriving, Kid Buu battles with Goku, quickly gaining the upper hand, causing Goku to transform into Super Saiyan 3 (in the manga, he does this right away instead of fighting in the Super Saiyan 2 form) and fight evenly with him.
Majin Buu's true form, "Kid Buu".
Kid Buu, having the advantage of perpetual strength, wins over the battered Goku, who is quickly losing power from his Super Saiyan 3 transformation. Vegeta transforms into his Super Saiyan 2 form to fight Kid Buu in an attempt to buy some time, but is quickly defeated as well. 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 Fat Majin Buu. His frustration ultimately leads him to spit out a small stone which explodes into an unconscious Fat Majin Buu. Now with nothing to stop him, Kid Buu resumes the play-fight with the enraged Mr. Satan, and attempts another pitiful attack when Fat Majin Buu steps in. Fat Majin Buu angrily tells Kid Buu to leave Mr. Satan alone and that his battle is with him. Fat Majin Buu and Kid Buu then engage one another in combat. The battle quickly turns in Kid Buu's favor, and he ultimately dominates Fat Majin Buu, who however, refuses to give up.
Meanwhile, Goku and Vegeta hatch a plan to have Dende and the Supreme Kai's use the Namekian Dragon Balls 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 Spirit Bomb, while Vegeta uses telepathy to ask for the Earthlings' help, to which only their friends and family respond.
Fat Majin Buu and Kid Buu continue their battle, until Fat Majin 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 time. Ultimately, Mr. Satan manages to convince the people of Earth to give up their energy, and the Spirit Bomb reaches completion. 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, Fat Majin Buu wakes up, and smashes himself headfirst into Kid Buu, knocking him off Vegeta for Mr. Satan to carry off to safety.
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.
With Kid Buu gone, the healed Vegeta angrily prepares to finish off the unconscious Fat Majin Buu, not wanting to risk the creation of another Evil Buu through Fat Majin Buu's anger. But Goku believes that Fat Majin Buu has redeemed himself and deserves to live the life of peace that he craved, and asks Dende to heal Fat Majin Buu. Kibito Kai then uses his power to teleport Goku, Vegeta, Dende, Mr. Satan, Fat Majin Buu, and Bee back to Earth, where they are given a warm welcome by their friends and family.
In order to settle things for good with Fat Majin Buu, Goku wishes to erase all the Earthlings' memories of Majin Buu. Mr. Satan also gives Fat Majin Buu strict orders to refrain from getting angry, no matter what.
Buu's only reason for existence was to cause absolute destruction. He enjoyed the destruction he brought to the universe. On the rare occasion he ever showed any restraint of his power, it was to make his "game" of destruction last longer instead of producing an instant kill. He is not a sadistic killer seeking to bring pain to his victims, as he seemingly doesn't know or even care about pain, nor does he seek to lord over anything. He merely seeks to have fun creating havoc and destruction in the whole universe. Only by Kid Buu gaining enlightenment through absorbing other beings is existence spared, and, of course, by a Genki Dama fueled by beings from across the universe and beyond.
Though he is highly irrational and insane, Kid Buu does seem to have some intelligence, as he quickly realized that Goku refused to hit him with the Spirit Bomb as long as Vegeta was in the area and took advantage of it by pinning the latter to the ground.