Kid Buu Character Facts

Kid Buu's personality is very similar to that of a very young child, in that he retains no form of compassion or remorse for any of his actions. Furthermore, due to his child-like personality and mind-set, he is incapable of developing empathy and comprehending the nature of his actions. Kid Buu is also, like a child, very unpredictable in his behavior and actions, sometimes randomly falling asleep during battle, or acting like a monkey by banging his chest with his fists. 
Buu's only reason for existence was to cause absolute destruction. He enjoyed the destruction he brought to the universe, reveling in the devastation he wreaks. He appears to enjoy testing the limits of his own power as well, seeking out Vegeta and Goku just to see how far they could push him. He is also utterly merciless, annihilating entire planets simply because Vegeta and Goku were not there. On the rare occasion he ever showed any restraint of his power, it was to make his "game" of destruction last longer instead of producing an instant kill. He is not a sadistic killer seeking to bring pain to his victims, as he seemingly does not care, or even know about pain, nor does he seek to conquer and rule over anything. He merely seeks to have fun the only way he knows how; creating havoc and destruction in the whole universe, similar to the fat version of Buu before Mr. Satan told him to stop killing. 
Strangely, despite his destructive tendencies, Kid Buu seems to have a pure heart, as he is able to deflect Goku's Spirit Bomb back at Goku when it jams (King Kai earlier states that only someone with a pure heart could deflect the Spirit Bomb, though one being pure evil of heart may also be able to deflect it). His child-like personality is most likely what enables him to deflect the Spirit Bomb. 
Though he is highly irrational and insane, Kid Buu does seem to have some reasoning abilities and intelligence, as he quickly realized that Goku refused to hit him with the Spirit Bomb as long as Vegeta was in the way and took advantage of it by pinning the latter to the ground.
The original Kid Buu absorbs Southern Supreme Kai, creating a taller and incredibly muscular version of the creature. In addition, Buu's antenna also grows significantly larger. In this form his voice also changes from the previously high-pitched voice Kid Buu had, though he still does not talk much (apart from yells and grunts similar to Kid Buu). After a second absorption of the plump and good natured Grand Supreme Kai, Buu gains the deceptively harmless and child-like form first seen in the series.