Baby

The Tuffles were a technologically advanced civilization that shared Planet Plant with the primitive Saiyans. When the Saiyans revolted and wiped out the Tuffles under a full moon, the dying Tuffle scientist Dr. Myuu engineered Baby as a biological weapon of vengeance. Baby is a parasitic Neo Machine Mutant who can enter a host's body through open wounds, taking complete control of their mind while enhancing their physical abilities.

After possessing several fighters, Baby ultimately takes control of Vegeta, combining Tuffle technology with Saiyan power to create Baby Vegeta. In this form, Baby uses the Black Star Dragon Balls to wish the Tuffle homeworld back into existence and begins converting Earth's population into Tuffle loyalists through his parasitic eggs. Goku, having regressed to a child's body due to a wish on the Black Star Dragon Balls, is initially overpowered but achieves Super Saiyan 4 with help from Pan and the Golden Great Ape transformation. The climactic battle ends with Goku blasting Baby out of Vegeta's body and destroying him as he attempts to flee in a spaceship.

Baby expands on a piece of Saiyan history that was only briefly mentioned in Dragon Ball Z. The conflict between the Tuffles and Saiyans adds moral complexity to the Saiyan race, reframing them as conquerors who committed genocide against a more advanced but physically weaker species. Baby's quest for revenge is understandable even if his methods are monstrous, making him one of GT's more layered villains.

His possession of Vegeta is particularly effective from a narrative standpoint, as it forces Goku to fight one of his closest allies while trying not to destroy the body his friend inhabits. Baby also serves as the catalyst for Goku's transformation into Super Saiyan 4, the signature transformation of GT and a fan-favorite design that returns Goku to his adult body while incorporating Great Ape features.