Uub is the incarnation of Kid Buu. He is human and is originally from a tropical island. He is the eldest of five siblings. His name, Uub, is Buu spelled backwards. This is due to the fact that he was born to be the positive incarnation of the evil Kid Buu.
At the age of ten Uub decides to enter the 28th World Martial Arts Tournament, in hopes of being able to win the prize money for his family. As per request by Goku, Mr. Buu changes the matchups so that he is able to fight Uub in the first round.As Uub begins his fight with Goku, he at first seems very reluctant to fight. Although, this quickly changes after Goku begins to insult his parents. Uub's power level then increases to a level that would be able to succesfully inflict damage on Goku. After the match, Goku apologizes and offers to take Uub in as his student. Uub, at first, is reluctant to take Goku up on his offer due to the fact that the reason he entered the tournament was to help out his family with the prize money. Although, Goku then explains that he will simply just have Hercule give Uub all the money he needs in order to support his village. After hearing this, Uub agrees and he and Goku fly off to his village to train. This marks the end of Dragon Ball Z. At the beginning of Dragon Ball GT, Uub is now a teen and is seen fighting with Goku on top of Kami's Lookout. His clothing now resembles that of Goku's at the end of Dragon Ball Z and his appearance has changed. He has gained ample amounts of muscle and no longer looks malnourished. At the conclusion of their fight Goku states that Uub's training is now complete. After saving Pan and witnessing Goku's supposed death at the hands of Baby, Uub decides to continue the fight his master could not finish. Although, Uub proved to be no match for Baby, and soon found himself on the receiving end of Baby's Revenge Death Ball. Before Uub could be destroyed, he was saved by Mr. Buu. The two of them then decided to fuse together in order to increase their strength. Together they are able to match the power level of Baby, though they are turned into chocolate shortly after.
Though Uub was assumed to be dead, it turns out it was not the case. He was able to succesfully stop Golden Great Ape Baby Vegeta from attacking Goku, by attacking him by the insides. He was then spit out by Baby to stop the pain. Majuub later fights in the 31st World Martial Arts Tournament, throwing the match in the final round facing Mr. Satan.
During the Super 17 Saga, Majuub is seen fighting against clusters of dead enemies as well as Super 17. Later Majuub contributes during the fight against Omega Shenron and is rather successful in being able to hold him off.