Boo is introduced in the series as an innocent looking, pudgy, pink genie-like creature. Later in the series, it is revealed that Boo's original form is a more sleek child form ("The Boo of Pure Evil"), however his nature and form had been altered after he'd assimilated the gentle, pudgy god (Dai Kaioshin) into his body. Boo's creator, a wizard named Bibbidi, had used Boo to wage war on the Kaioshin. Boo managed to assimilate two of the four Kaioshin, the second being the one who altered his nature to become innocent. After Bibbidi sealed Boo away, Bibbidi was killed by the fourth Kaioshin, and Boo was transported to Earth where he layed dormant for five million years until he was revived by Bibbidi's son Babbidi during the events of Dragon Ball.

Boo has numerous counterparts in the series. When he first emerges from his shell, he is tall and round, and his personality is naive and innocent. Boo's anger later manifests itself in the form of another Boo ("The Boo of Pure Wrath"). After a battle with the Boo of Pure Wrath, Boo is absorbed and the two combined create Super Boo, who tracks down the protagonists of the series Piccolo and Gotenks. He later reverts to his purest form, the original state called "The Boo of Pure Evil".