Oob

Oob is a child that was introduced in the World Tournament after Goku defeats Majin Boo. When Boo is killed, Goku wishes that he gets reincarnated as a good person so that he could battle with him once again. Oob is the result of this wish, hence his name. Goku decides to train Oob at the end of Z and GT begins with the completion of Oob's training. Oob grew up a young warrior/farmer boy. Oob got his particular powers because he was created from Boo. Oob later fuses with the original Majin Boo. This transformation doesn't affect his appearance much, but increases his power to the point of almost matching Bebi in power. He then later fights Super 17 and then later the evil dragons.

(Source: Wikipedia)