Majin Buu is a formidable and ancient entity from the anime Dragon Ball Z Kai. Originally an embodiment of pure chaos and destruction, his nature is significantly altered after absorbing a benevolent deity, leading to the creation of multiple distinct personas throughout the story. His origins are not as a created being but as a primordial force of nature, a being who has existed since time immemorial and was merely awakened by the evil wizard Bibidi rather than being manufactured by him. This original, untamed form is known as Kid Buu, a small, lean, and incredibly vicious creature driven solely by an insatiable appetite for combat and annihilation, with no capacity for reason or control.

The character's journey begins in earnest when Kid Buu is unleashed upon the universe by Bibidi. During a genocidal rampage across the galaxy, Buu kills two of the five Supreme Kais and then proceeds to absorb two others. The second of these is the Grand Supreme Kai, a gentle and rotund deity. This absorption has a profound and transformative effect, merging the pure evil of Kid Buu with the kind-hearted nature of the Grand Supreme Kai. The result is the first version of Buu seen in the series: a fat, childlike, pink creature. This "Innocent" Buu, while still possessing monstrous power, is calmer, easier to control, and develops childlike personality traits, including a love for sweets and a tendency to refer to himself in the third person.

Personality and motivations shift drastically between Buu's different forms. The fat, innocent Buu is playful, naive, and easily amused. He initially destroys things because he has been commanded to do so by his master, the wizard Babidi, treating the destruction of cities and the annihilation of populations as a fun game. He shows no inherent malice but rather a complete lack of understanding of morality or the value of life. This changes when he befriends the Earth's champion, Mr. Satan. Through this simple friendship, Buu learns that killing is wrong and vows to stop, demonstrating a capacity for change and loyalty. However, when this innocent Buu becomes enraged by the shooting of his friend and his pet dog, his anger manifests physically, expelling all the evil from his body. This creates two separate beings: the now-pure "Good Buu" and a thin, grey "Evil Buu." The Evil Buu quickly overpowers and absorbs the Good Buu, transforming into a new, dominant form known as Super Buu.

Super Buu represents a terrifying middle ground. He retains the intelligence and speech of the innocent Buu but is far more cunning, ruthless, and powerful. His primary motivation is the lust for battle and a sadistic enjoyment of terrorizing and destroying his opponents. He is strategic, able to learn techniques after seeing them once, and possesses a cruel sense of humor. His role in the story is that of an overwhelming, unstoppable threat who systematically hunts down the Earth's remaining defenders. He absorbs powerful fighters like Gotenks and Piccolo, and later Gohan, to increase his own strength and take on their physical characteristics and abilities, making him progressively more formidable. Key relationships at this stage are purely antagonistic; he is a force to be eliminated, leading to the desperate fusion of Goku and Vegeta into the warrior Vegito. After the absorbed heroes are freed from inside Super Buu, his body rejects the Good Buu he had previously absorbed, causing him to revert to his original, pure form: the mindless Kid Buu.

Kid Buu is the final and most dangerous incarnation. Stripped of the Grand Supreme Kai's influence, he is a raw hurricane of destruction. His motivation is nothing more than to destroy for the sheer joy of it. He teleports to the Sacred World of the Kais and immediately destroys the Earth with a single, massive energy ball. Unlike his other forms, Kid Buu does not strategize or talk extensively; he fights with manic, unpredictable glee and can regenerate from any damage, fighting without fatigue. In the end, he is so purely evil that he is particularly vulnerable to the Spirit Bomb, a technique that draws upon the righteous energy of all living things. This is the only attack powerful enough to finally obliterate him.

Buu's development is unique in that his character arc is circular. He begins as a pure evil force, becomes a neutral, childish being capable of good, splits into good and evil halves, and ultimately regresses back to his original evil form before being destroyed. His legacy, however, includes a continuation of his good side. The original fat, innocent Buu is spared after Kid Buu's defeat. He remains living on Earth as "Mr. Buu," a friend and ally to the Z-Fighters, particularly Mr. Satan. Goku also wishes for Kid Buu to be reincarnated as a good person, resulting in a human boy named Uub, who becomes his apprentice.

Notable abilities are what make Majin Buu such a legendary threat. His body is composed of a magical, gum-like substance that grants him incredible powers. His most infamous ability is regeneration: he can survive being blown to pieces, incinerated, or reduced to mere vapor, reforming his body almost instantly without any loss of stamina. He also possesses a powerful transmutation magic, typically fired from the antenna on his head, which allows him to turn any object or person into whatever he desires, most commonly chocolate or other candy, which he then eats. His body can stretch, shapeshift, and detach pieces of itself to engulf and absorb other beings into his own form. This absorption ability is his most dangerous tactic, as it allows him to permanently add a target's power, knowledge, and physical traits to his own, making him stronger with each victim he assimilates. Other abilities include near-limitless strength, energy sensing, the power to create dimensional tears with his scream, and the capacity to learn complex techniques after seeing them just once.