Dragon Ball Z ended with the sprawling Buu Saga, which had the Z Fighters facing their most powerful opponent yet. Eventually becoming the diminutive but impressive Kid Buu, Majin Buu is first seen in a much more corpulent, playful form. This Buu still has the power to back up his reputation, but it's held back by a childlike mentality.
Ironically, one of Buu's friends by the series' end is none other than Hercule/Mr. Satan. Usually more interested in self-aggrandizing "heroics," Hercule ultimately helped save the day through his strange bond with Majin Buu. Here's how becoming best buds made both Buu and Hercule better people in Dragon Ball Z.
How Hercule Met Majin Buu
While Buu grows tired of his overbearing master Babidi and soon kills him, Hercule is charged with taking the pink monster out for good and saving the human race. Unfortunately his feeble attempts, such as trying to feed Buu poisoned candy and giving him an explosive video game, fall completely flat. Buu is so powerful that he doesn't even view the acts as threats, which thankfully keeps him from seeing Hercule as one. Eventually, Buu finds a hurt puppy that he and Hercule name Bee. Together they nurse Bee back to health and bond over the animal, with Hercule using the opportunity to illustrate how precious life is and teach Buu that mindless killing is wrong.
This new morality is tested when Hercule himself is shot, with Buu trying not to kill his attacker while also being incredibly angry over the attempt on his new friend's life. This sees Buu expel the evil within him, which becomes the ridiculously skinny and gray-colored Evil Buu. This proved a critical development in the Buu Saga, and Hercule was instrumental in its occurrence.
Buu Friends Forever
Majin Buu was once a purely evil creature, but consuming the Grand Supreme Kai long ago gave him a fatter, more jovial form that also put some good into his soul. Hercule ultimately cultivated these kind instincts, namely in relation to protecting innocent beings such as Bee. For as selfish and arrogant as Hercule usually is, the concern he has for the dog and later Buu is genuine and rubs off on the latter.
When Evil Buu absorbs Good Buu and becomes Super Buu, the mostly evil villain still remembers Hercule's friendship and spares his life. This helps keep Hercule around, and he's actually pivotal in defeating Kid Buu by rescuing Vegeta's battered body so Goku can throw a Spirit Bomb. Good Buu also holds his evil counterpart down to ensure Hercule gets away, giving the good guys the best possible chance of success.
This again reflects Buu's change of heart after befriending Hercule, making a once-joke of a character incredibly important to saving the world in his own way. It's also a reversal of one of the franchise's biggest tropes. Dragon Ball is known for Goku befriending former enemies such as Piccolo and Vegeta. In the case of Dragon Ball Z's final foe, however, it was the most innocuous, seemingly unheroic human who helped a bad guy see the light. This would see both Hercule and Good Buu become recurring characters in Dragon Ball Super, and it's all thanks to their mutual love of massages, a certain dog and a timeless bromance.