Although he is one of the most powerful entities, Majin Boo is often playful and displays his childish personality, revealing he destroys things for fun simply from old instructions from his master, stopping once told what he was doing was wrong thanks to the Grand Supreme Kai's personality showing in him. Though he is playful and silly, his evil side is readily apparent, especially shown when his eyes widen, and steam shoots out of the holes on his body. His extreme anger issues show themselves often, when he is even slightly made fun of, or starts to lose in a battle. He finds enjoyment in taking the lives of many and shows no remorse, though this is because he treats everything as a game. In battle, he fights in a bullying manner, using his immense size to outflank his opponent. Once he was absorbed by Evil Boo, his playful and childish traits manifest in Super Boo and Kid Boo respectively. During his second battle against Gotenks, Gotenks was preparing the Super Ghosts of his Super Ghost Kamikaze Attack for battle. Piccolo showed Gotenks that Boo had already regenerated from the last Super Ghost attack. Super Boo was also lying on his stomach, drinking a soda (though it looked like a milkshake) and reading a magazine, much to Gotenks' fury and embarrassment. Also, when Goku and Vegeta were preparing for battle, Kid Boo was sleeping (as Boo only needs so little time to rest), much to Goku's surprise. More importantly, due to his connection with Mr. Satan he had influence to stop him from killing the former as Super Boo and later Kid Boo.  For this reason, he also responded to Videl's request to be patient due to being Hercule's daughter; though it wasn't enough to stop him from killing her. While mildly annoying to Super Boo, it infuriated Kid Boo enough that he spat him out as soon as he was able to.

Buu also possesses an enormous appetite for sweets, shown to far exceed even Goku's eating habits, as Boo was still hungry after he once turned a whole city of people into candy and ate them. Majin Boo only has to sleep for about five seconds to get the results humans get from hours of slumber. He is extremely naive, as he was completely ignorant of Mr. Satan's (inept) attempts to kill him. Despite the pleasure he takes in mass-murder, it is later revealed that he only committed such acts because he was unaware of the moral implications of his actions, and because he had been commanded to do so by Babidi, and Bibidi before him. The Grand Supreme Kai's presence is more influential in this form than in Super Boo, who is mostly evil and is influenced very little by the Kai. He literally struggles with the evil inside himself, with the evil winning and usurping near complete control, Majin Boo becoming far more dangerous as a result.