Kid Buu= original Buu aka pure evil buu with no reason. Wayyyy back OG buu was pure evil was destroying planets/galaxies and what not. The Kais(not King Kai but the Supreme ones) all tried to kill him, they all failed cept for this fat nikka right there who was the grand supreme Kai
He was a match for buu and I guess Buu couldn't kill him so he absorbed him with one of his body parts laying round somewhere near him. When that did well it OG buu became fat buu and since the Supreme Kai was that good(in terms of good/evil) it kinda softened him up a lil bit. Now Babidi put this nikka fat buu to sleep. Fat Buu came back like as well all know in DBZ. He eventually killed Babidi. Had his own crib and Satan(human guy the fraud from the championships that got washed by cell in a swipe haha) became friends with him. He became even more good and when some nikkas that was tryna kill buu shot Satan in the head Buu almost snapped but instead just ejected all that evil out of him which gave him the skinny buu. He tried to fight Skinny Buu but Skinny Buu won and ate him and it led to this
Now a whole lotta shyt happened he absored gohan picolo and gotenks. Vegeto(fusion of Goku and Vegeta with the potaras) got absorbed but he planned it on purpose in order to retrieve gohan picolo and trunks and goten. Goku and Vegeta took all of em out and also detached Fat Buu(good buu) from there and it led back to the OG kid Buu. now OG kid buu fought Goku and lost on the Supreme Kai's planet and before Goku offed him with a Spirit Bob he said he wished that he could have fought him under different circumstances or some shyt like that. Someone in the Afterlife heard him and well he was Reincarnted in Ubb(black kid)