From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Evil Boo (魔人 ブウ 純粋 悪, Majin Bū Junsui Aku; Viz "Evil Djinn Boo"; Literally meaning "Majin Boo Pure Evil") is a Majin.
Evil Boo was the tall, thin, gray, and emaciated form who appeared after Fat Boo dispelled the evil from within body. Evil Boo represented the dark side of Fat Boo and possessed the bulk of their power, a result of Fat Boo's anger becoming more than he wanted until he expeled it as Evil Boo.
This explained why his reaction to the shooting of Mister Satan was so despicable and even appalled the Fat Boo. Evil Boo was a polar opposite of Fat Boo in all aspects apart from apparel (except for him having a dark blue cape where Fat Boo had a purple cape), including the color of his flesh (a drab grey), his incredibly thin body, and gravelly manner of speech.
 Dragon Ball Z
Evil Boo immediately proceeded to mercilessly eliminate crazed gunman Van Zant in the mountains. He then battled against Mister Boo, but the evil incarnation quickly gained the upper hand and consumed Mister Boo after he reflected an attack meant to turn him into chocolate. Evil Boo then ate the chocolate Mister Boo causing them to re-fuse, this time with the evil on the outside instead of the inside, into a new form, Super Boo.
 Video games
Evil Boo appeared as a playable character in the Budokai Tenkaichi video game series as Majin Boo (Pure Evil). He could use the Kamehameha, but did not know the Chocolate Beam and his stats were very similar to Fat Boo's stats.
He also appeared in Dragon Ball Z: Buu's Fury.
Because Evil Boo is directly split from Fat Boo, it can be assumed that he possesses the same techniques. One of the few techniques that Evil Boo is seen using is Fat Boo's Vaporize! technique, though it is reddish-orange and is performed in the reverse order. Evil Boo charges up the energy sphere and fires it in the form of a large energy wave. He uses the attack to kill Van Zant and also destroys the entire mountain the gunman is on at the time.
 Voice actors
- Japanese dub: Kōzō Shioya
- Ocean Group dub: Brian Dobson
- FUNimation dub: Justin Cook
- Latin American dub: Mario Sauret
- Evil Boo's voice is high-pitched in Dragon Ball Z: Budokai Tenkaichi. This is later fixed. Evil Boo still only yells and grunts in the Budokai Tenkaichi games despite being able to speak in the series.
- Evil Boo is the only Boo form to refer to himself with correct grammar (without the absorption of Piccolo). He says, "I'm Majin Buu" instead of "Me Majin Buu" or simply "Buu" like all other forms. This could mean he is the most intelligent of the four main forms.
- He is the only form of Boo who does not to display any stretching of his body parts as well as the shortest lived Boo, his appearance being little over half an episode in the anime. He is also the only incarnation of Boo to not have pink skin.