The character we are talking about is none other than boo . boo is one of the versatile characters ever found in dragon ball series , the character boo was first introduced in dragon ball z , where he was guided by margin , a small akward looking character who could perform magic and control people , by then boo was this fat and cute looking person who loves to play and by play we mean hard core fight .
This love of boo towards fight had made him so mad that he kills his mentor and master and tries to fight with goku and friends , in this process the bad part of boo turns up and he looks a lot slimer than boo himself and when boo tries to absorb him the evil boo absorbs boo himseld and becomes much stronger .
Later proceeds and absorbs anyone tho tries to fight him i.e., gotens , bulma , che-che and other strong fighters , when this happens goku asks gohan to fight boo , at this time gohan had been trained to take on boo with the help of a kai who appears in the series earlier to that episode , but unfortunately gohans training goes to waste cause , when gohan doesnt concentrate his towards the power around him a part of evil boo attacks and absorbs gohan completely .
This is when boo gets a whole lot stronger , since he cant control his powers in his present state he transforms into kid boo which is his last form , until goku finally appears on the scene to take on boo and transforms into ssj3 and vapourises kid boo with a golden fish or also called as spirit bomb .
Thanks to goku’s inteligence at the right moment goku makes kid boo to spillout all others he had absorbed that includes gohan , bulma , che-che and others …
After kid boo is vapourised goku wants to fight him again so he requests kai to provide a rebirth to kid boo in an humanly form called uub .