Uub when first introduced in the Kid Buu Saga, is very timid in his personality, despite his amazing power, becoming frightened at someone's slight expression. However, after training with Goku for many years, he becomes a confident fighter, wanting to make sure all evil is vanquished.
Uub is from a tropical island, and he grew up in a small village on the island. He is the oldest of five siblings and he works hard to take care of his family who are starving and poor.
Dragon Ball Z
|“||I'm afraid that what mother said is true... Even though I'm the strongest one in my village, there may be some men in this world that are even stronger than me... All these men look so powerful...||„|
|~ Uub in Buu's Reincarnation|
As a young boy, at the age of ten, Uub competes in the 28th World Martial Arts Tournament, which takes place ten years after the defeat of Kid Buu, wanting the prize money to help his family who are starving and poor. It is revealed here by Goku that Uub is actually the reincarnated Human form of Kid Buu, and that he was born from the wish made by Goku just before he defeated Buu with the Super Spirit Bomb.
Uub actually has good intentions of entering the World Tournament; his plan is merely to attempt to win the prize money of 10,000,000 zeni in order to feed his starving poor village. Being the strongest from his village, Uub then realizes that there may be 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, which was changed by Majin Buu at Goku's request, 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, insulting his parents. Uub's power increases to a point of being able to hurt Goku, although during the fight Goku never goes Super Saiyan. 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 will have Mr. Satan give Uub all the money he needs. They both then head to Uub's village to train, which is the final scene of Dragon Ball Z. He is trained by Goku and becomes a warrior within the Z Fighters, helping to combat enemies during Dragon Ball GT.
Although the Z Fighters and other tournament participants initially perceive Uub as frail and inexperienced, the more he would fight against Goku in his provoked and angered state, the more he would adapt to Goku's attacks and grow in power. It is heavily implied that by the end of their fight, Uub's power was equivalent to Goku's, who had not only acquired the most powerful Saiyan transformation before the ten year skip in time, but had spent the last decade training before the tournament. Furthermore, the once-timid boy was capable of producing powerful, invisible waves of ki that could uplift the arena's stone foundation, and was his signature attack aside from his basic punches and kicks.
Dragon Ball GT
Black Star Dragon Ball Saga
|“||Well, Goku, I can't tell you how much I appreciate everything. You changed my life!||„|
|~ Uub to Goku before departing|
|“||He was my mentor and my friend! He taught me what I knew so that one day I could use it and make things right. And I will!||„|
|~ Uub while confronting Baby Vegeta|
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, who 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 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 it is assumed he is dead, when Baby Vegeta turned into a Golden Great Ape he was revealed to be alive when Uub stopped Baby from attacking Goku, announcing that he'd purposefully let himself be eaten so he could attack 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 defeat Baby using the full power of a Super Saiyan 4.
Super 17 Saga
Majuub appears in this saga while fighting in the 31st World Martial Arts Tournament under 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 while Goku was trapped in Hell, until he is defeated by the 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 is greatly weakened by his encounter with Omega Shenron. 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 rest of battle against Omega Shenron on the sidelines.
Majuub stalls Syn Shenron while Goku gathers energy from the Z Fighters 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 have 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 is formed that is powerful enough to keep Goku steady to finish off the last of the Shadow Dragons. While Majuub does not contribute energy to the attack, he does 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 if it is ever in danger again.