"You'll only fight one of us, because after what I've got for you, I make you one promise...you won't be able to tell up from down!"
— "Shadow Dragons Unite"
Uub is the good reincarnation of the Kid Buu. He is also arguably the strongest human in Dragon Ball Z franchise.
During the post-Kid Buu Saga, Uub went to enter the World Martial Arts Budokai to win money and help his starving village and ends up fighting Goku. Before the fight, Goku identifies the kid as Buu's incarnation and becomes convinced to showcase this power. He taunts the young warrior and eventaully Uub brings out his inner potenial. Fighting beautifully now, Uub proves to be a match for Goku. Goku realizes that Uub still has to control his power since he can't fly yet so he decides to train him.The kind-hearted Saiyan flies away with his new student and future protecter of the Earth.
Dragon Ball GT
Black Star Dragon Ball Saga
Five years (ten in the dub) after the 28th World Tournament, Uub is now a 15-year-old teen (20-year-old adult in the FUNimation dub) and is seen training with Goku inside Kami's Lookout. The training ends, and Goku tells Uub he is done with his training. Uub wishes him farewell and flies 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 muscular and toned, instead of skinny and malnourished as in Dragon Ball Z.
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 seemingly slain at Baby Vegeta's hands (Goku was teleported out by Kibito Kai, 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, resulting in increased power and Buu's signature ability to turn enemies into candy (though Majuub does this with his finger, rather than an antenna on his head). He ends up matching Baby Vegeta in power, but is later on turned into chocolate and eaten by Baby Vegeta.
Though he is assumed dead, when Baby Vegeta turned into a 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 bought Goku enough time to gather power from Gohan, Goten, Trunks, and Pan, allowing him to effortlessly defeat Baby Vegeta.
Super 17 Saga
Majuub after he let Mr. Satan win.
Majuub appears in this saga while fighting in the 31st World Martial Arts Tournamentunder the disguise of Papayaman. Making it to the finals, Majuub is prepared to beat Mr. Satan and become the new world champion until Majin Buu telepathically forces him to throw the fight, so that Satan will still be world champion and the people's hopes will remain high. Majin Buu refers to Mr. Satan as "The World's Hero." Majuub pretends to lose to Satan, but unlike Android 18, Majuub does not 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.
Shadow Dragon Saga
In the battle against Omega Shenron, after Goku (in his Super Saiyan 4 form) is defeated by the Shadow Dragon, Majuub intervenes along with Trunks, Gohan, and Goten. Due to being no match for the superpowered 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 fuse, 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 (Goten, Trunks or Gohan), 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.