Uub is the reincarnation of the evil Majin Buu. After Buu is destroyed, Goku asks him to be reincarnated as a good person, so they may battle again. King Yemma overhears the request and complies with it. Goku eventually senses Uub's hidden power, and seeks him out when Uub attends the tournament to gain money for his village. Goku ends up deciding to train him to be Earth's new guardian and as an opponent that he can battle. In Dragon Ball GT, Buu merges with Uub to become "Majuub", to face threats such as Baby, Super #17, and the Shadow Dragons.
As a young boy at the age of ten, Uub is first seen at the 28 World Martial Arts Tournament which takes place ten years after the defeat of Kid Buu. It is revealed here by Goku that Uub is actually the reincarnated human form of Kid Buu, who was born thanks to the wish made by Goku just before he defeated Kid Buu. Uub, actuality, had good intentions of entering the World Tournament; his plan was merely to attempt to win the prize money of 10,000,000 zenie in order to feed his poor village. Being the strongest from his village, Uub then realizes that there maybe people in the world who are even stronger than him. With this, Uub loses hope, and becomes even more nervous when taunted by Nok.
After the draw, Uub is set to challenge Goku in the first round, but is nervous and even flinches when Goku takes a step toward him. During combat with Goku, he regains his confidence and power, after Goku purposely provokes him. Uub's power increases to a point of being on par with Goku's, although during the fight Goku never goes Super Sayian. After Goku and Uub's match Goku, apologizes then offers to train Uub. At first Uub is worried about the prize money but Goku says he'll have Hercule give Uub the money he needs. They both then head to Uub's village to train.
Ten years after the 28 World Tournament, Uub is now a 20-year-old adult and is seen training with Goku on Kami's Lookout and fighting in an immense battle inside the Hyperbolic Time Chamber. The battle ends, and Goku tells Uub he is done with his training. Uub shakes Goku's hand goodbye, and flies off, back to his village. Uub's appearance is now different because of the ten years of training. He now wears a gi much like the one Goku wore when they first met. He's also much more filled out because of the long time training, having a very muscular body shape instead of the skinny one he had when last we saw him.
When the Earth is attacked by the Tuffle parasite known as Baby, all of the Earthlings are controlled by Baby's powers and become his followers. Gohan and Videl, whom are possessed by Baby, confront Pan in order to kill her. She is rescued by Uub, who confronts the two and stops them from harming Pan.
Seeing that Goku, his trainer and friend, has been slain at Baby Vegeta's hands (Goku was teleported to Supreme Kai's Planet, but was presumed killed), Uub faces off against Baby, but Baby easily dominates Uub and pounds him. Now down, Uub can do nothing but watch Baby prepare to finish him off with the same attack he used on Goku: the Revenge Death Ball. Until he is saved by Majin Buu. Uub then fuses with Majin Buu and becomes Majuub but later on gets eaten by Baby Vegeta.
Though he is assumed dead, when Baby Vegeta turned into an Golden Great Ape he was revealed to be alive when he stopped Baby from attacking Goku, via attacking the monster from the inside. Baby spits him out to overcome the pain, but not before Majuub had bought Goku enough time to get powered by Gohan, Goten, Trunks, and Pan, allowing him to effortlessly defeat Baby.
Majuub appears in this saga while fighting in a World Tournament under the disguise of Papayaman. Making it to the finals, Majuub is prepared to beat Hercule and become the new world champion until Majin Buu telepathically asks him to throw the fight, so that Hercule will still be world champion and the people's hopes will remain high. Majuub pretends to lose to Hercule, but unlike Android 18, Majuub doesn't ask for anything in return.
When a rip between Hell and Earth appears, Majuub goes into action and begins to fight off some of the dead villains, such as Saibamen and General Rilldo who returns from Hell. He then fights Super 17 along with Goten, Trunks, Vegeta and Gohan, until he is defeated by the ultimate android.
In the battle against Omega Shenron, after Super Saiyan 4 Goku is defeated by the Shadow Dragon, Majuub intervenes along with Trunks, Gohan, and Goten. Due to being no match for the super powered Shadow Dragon, Majuub is solely able to stall Omega Shenron for a little while in order for Goku to be able to reach Super Saiyan 4 once again. Unfortunately, Majuub is only able to hold Omega Shenron in a half-nelson for a few seconds until he is blasted in the face and knocked out by the Shadow Dragon.
But Majuub quickly recovers with the resilience inherited from his fusee, though his ki was greatly weakened by his encounter with Omega. He wakes up just in time to witness Goku and Vegeta perform a Super Saiyan 4 fusion and form Super Saiyan 4 Gogeta. Impressed by the technique, Majuub volunteers to fuse with anyone available (Trunks, Gohan, Goten, etc.), but Gohan explains to Majuub that it requires several hours of practice and that their power levels must be equal, and Majuub was not at his peak, due to Omega Shenron's attack. Majuub then watches the battle against Omega Shenron on the sidelines.
Eventually, Majuub is of little help in the climatic showdown, merely helping getting Chi-Chi to safety when the remaining Saiyans decide to avenge Goku. Presumably if Goten, Trunks, Gohan, and Vegeta failed, Majuub would've been the only hope left of taking down the final dragon. However, due to Goku's miraculous and very ambiguous survival of the Minus Power Ball, a universal Spirit Bomb was formed that was powerful enough to keep Goku steady to finish off the last of the dark Dragons. While Majuub didn't contribute energy to the attack, he did bear witness to the climax of the battle.
After Goku's departure with Shenron, Majuub bids farewell to his friends and returns to his village. It is likely that since Goku is gone, he will continue training extra hard, in order to defend the Earth again, if it is in danger.
Even before his training with Goku, and his subsequent merge with Majin Buu, Uub manifested incredible physical prowess when he first encountered Goku at the World's Martial Arts tournament. Once he was able to tap into his potential, his speed, strength, and durability rivaled that of Goku's Super Saiyin 1 form. Uub's powers increased once he finished his training with Goku at Dende's Lookout, and then again when he merged with Majin Buu to become Majuub. His power levels are greater than any human has obtained in the various DragonBall series, since his martial arts skill, energy manifestation abilities, and superhuman physical prowess exceed Goku and Gohan's Ascended Super Saiyan Forms (Class 100 Strength, Supersonic Flight Speed and reaction time, Planet-destroying Energy Attacks, etc)