After episode 1, Uub won't make an appearance for a while. But there is a fighting scene regardless. So you must be able to scream/yell. Uub is the reincarnation of the evil Majin Buu. However, he's as good as they come. He's Goku's pupil in the beginning, and later on in the series, he'll end up fusing with Buu to fight Baby. The ongoing joke with Uub is, that he's poor and everyone's trying to get him to pay for stuff. 

For the voice, you can do whatever you like, as long as it fits the character description and age. He must sound youthful.