Uub is the reicarnation of Kid Buu and one of the support characters in DBZ and BH's Dragon Ball GT, he doesn't apear in Gohan's universe nor Vegito's universe, and his possible existence in Gokule's universe is an mistery
DBZ[]
10 years after the defeat of Kid Buu by Goku's Spirit bomb, Uub came as Goku wanted (Even thought Bardock, Gine and Turles already knew about him being the reicarnation of Buu via their being in hell at the time priod to their revival). Uub unaware of his potential was too shy to fight goku, so goku forced to be rude which brouth Uub to an full potential. After the fair fight, Goku leaves with Uub to train him.
DBGT[]
5 years have passed since goku went with Uub, BH eventually found Uub and revealed to him (via his alt universe self) that he was once the evil Kid Buu, this allowed Uub to be majined by the prince and be the enemy to the Z (GT?) warriors, althought the reicarnation of Buu returned to normal. Uub then fought againts Super 17 and Super 18 but got owned badly and witness the murderer of his teacher - goku, he then (after his master was revived) fought againts the Shadow Dragons and proved help againts Omega