Boo (Buu in the English anime and video games) is a fictional character in the manga Dragon Ball/Dragon Ball Z and the anime Dragon Ball Z and Dragon Ball GT.

Boo is the most powerful enemy faced by the Z Fighters in the series. While incredibly powerful himself, Boo gets a great deal of his strength by absorbing others, adding his victims' power to his own and he has also killed nearly every character. He absorbs by either turning the victim into food and eating them, or covering the victim with his slime and consuming them.

Boo was created by the evil wizard Bibidi many centuries ago and was used to destroy worlds.

Boo then attacked the Kaioshin, the gods of the universe. He killed North Kaioshin and West Kaioshin, then proceeded to absorb South Kaioshin, becoming the hulking Huge Boo. After nearly killing East Kaioshin, he then absorbed Dai Kaioshin and became Majin Boo (Majin Buu), a fat Boo who was kinder and weaker due to Dai Kaioshin's good nature.

As he became more and more difficult to control, Bibidi took to locking him inside a ball while traveling. While Bibidi was on his way to Earth, and Boo was locked in his ball, East Kaioshin attacked and killed Bibidi.

Bibidi had a son though, Babidi, and he figured out how to release Boo. Once released, Boo hardly seemed like a terrible villain, more like a naughty child. His power was not to be underestimated though, as he seemed impervious to assault and could turn even powerful warriors like Dabura into a harmless (and easily consumed) piece of food (which is probably a form of absorbing). Even Vegeta's devastating suicide attack on Boo only momentarily harmed him. He was split into hundreds of minuature Boo's, which then recombimed to become Boo again.

Boo eventually became sick of Babidi's bossing him around and destroyed him. He then makes friends with Hercule (Mr. Satan in the Japanese version), who makes him promise to stop killing, and even adopts a puppy. All seems well until two thugs shoot Hercule and the puppy. Boo, torn between his desire for revenge and his promise, becomes frustrated to the point that he releases steam from his head which condenses into another Boo. The new Boo, Evil Boo (Evil Buu), is thin, grey and has taken the bulk of Majin Boo's power. He is a stark contrast to the now good Mr. Boo (Mr. Buu), who looks the same as Majin Boo.

The two Boo's battle each other until Good Boo unleashes his conversion beam on Evil Boo. Being more powerful, Evil Boo simply reflects it right back. Good Boo is turned into a piece of chocolate and eaten by Evil Boo, resulting in Super Boo (Super Buu), a muscular being who has a green tongue, and an annoying habit of cracking his neck.

Super Boo then goes through several stages of increasing power. He first absorbs Piccolo, then Gotenks and finally Son Gohan. At this point he's at his peak of power, and nearly unstoppable. Vegeto managed to get inside of Boo and removed all of his absorbed victims (South Kaioshin and Dai Kaioshin are presumed to be part of Boo, so they aren't released).

This caused Boo to revert to his original form, Kid Boo, an unrepentant sociopath who lived to destroy and could not be reasoned with. Boo made a huge energy ball and destroyed Earth and only Goku, Vegeta, Dende, Hercule and Bee escaped with Kibtokai's help. Boo later appeared on the kai planet and fought Goku and Vegeta. Mr. Boo appeared again during the Kid Boo and Son Goku fight, being spat out before Kid Boo attacked Hercule. Mr. Boo was able to distract Kid Boo for a considerable time, helping Goku gather energy for his Super Genki Dama from the Earth. Hercule later convinced the people of Earth to give him the energy needed. After the defeat of Kid Boo, Mr. Boo was able to lead a "normal" life with Hercule and Bee.

In Dragon Ball GT, Mr. Buu fused with Uubu.