Main article: BuuMajin Buu is a demon created by the evil wizard Bibidi. He has the appearance of a fat, pink, rubbery creature with an antenna on his head. He can regenerate any lost limbs, heal any injuries, and even come back together if blown to bits. He can also turn anything into anything else, most often used to turn people into chocolate or candy.
Majin Buu originally looked identical to Kid Buu, but after killing the West Kai and North Kai at the time, he absorbed South Kai, and later Dai Kaio, turning him into the fat Buu. He was soon sealed away in a ball by Bibidi, who was then killed by Supreme Kai.
Bibidi's son Babidi set out to gather energy to awaken Buu, and succeeded by brainwashing Vegeta. He beat up Supreme Kai and supposedly killed Gohan, but Dabura, Babidi's right hand man, came to fight him, knowing Babidi would not be able to control Buu. Dabura speared Buu, but he pulled the spear out and healed the hole. He then fought Dabura and turned him into a cookie and ate him.
Afterward, Vegeta appeared and fought Buu, who soon gained the advantage, forcing Vegeta to blow himself up in an attempt to kill him. Buu survived, blown to bits, and came back together. He and Babidi set out to find Goten, Trunks, and Piccolo, with Buu turning the inhabitants of cities into candy or chocolate to eat them. When Babidi learned the location of Trunks' home, Goku stalled them and turned Super Saiyan 3 to fight Buu. Goku could have destroyed him, but wanted Goten and Trunks to in case a new villain arrived and they needed to fight him (as he was dead and could only go to Earth for 24 hours, once). After Trunks got the Dragon Radar, Goku left. Buu got tired of being abused by Babidi, so he killed him.
He then terrorized the world as much as he wanted to, turning people into clay and building a house out of them. Goten and Trunks fused into Gotenks to fight him, but got beat up. Hercule was sent to kill Buu, and used various means like poisoned chocolate and exploding videogames. Soon they became friends, and Buu found an injured puppy dog and healed him. When he found out from Hercule that killing innocent people was wrong, he decided to stop.
When Bee, and later Hercule, were shot by Van Zant, a deranged gunman, Buu was able to heal both, but became so angry that his evil side came out in the form of steam coming out the holes in his head (as usually does when he gets angry). The steam turned into Evil Buu, who beat up Majin Buu. Majin Buu tried to turn Evil Buu into chocolate, but he blew the attack back at him, turning him into chocolate and eating him, turning him into Super Buu.
When Goku and Vegeta were inside Super Buu's head, Vegeta tore down the original Majin Buu, turning Super Buu into Kid Buu. He stopped Kid Buu from attacking Hercule, so Kid Buu spit him out like a piece of chewing gum. He fought Kid Buu, but was badly beaten. After Goku killed Kid Buu, Dende healed Majin Buu, and everyone's memories of him (except the Z Fighters and their friends and family) were erased with a wish from the Dragon Balls, enabling him to live a peaceful life on Earth with Hercule and Bee.