Evil Buu is the darker, more powerful side of the Dragonball Z character Majin Buu. Dagonball Wikia explains that he is created by "splitting" from Majin Buu, as a direct result of unleashing his suppressed anger.
Evil Buu is the complete opposite of Majin Buu. While Majin Buu is a short, fat, pink Majin, Evil Buu is tall and thin, with a grayish skin tone. Majin Buu is also extremely childish, while Evil Buu has a much deeper, harsher tone when speaking. After a battle that causes Majin Buu to turn into a piece of chocolate, Evil Buu eats it, causing the two to merge again. This merging-action results in Super Buu, who has a similar physical appearance and mind of Evil Buu but the coloring of Majin Buu.