WARNING: The following article contains spoilers for the Dragon Ball Super manga, by Akira Toriyama, Toyotarou, Caleb Cook and Brandon Bovia, available now in English through Viz Media.
Dragon Ball has become famous for its characters' overly complex backstories, and the story of how Kid Buu came into being is one of the hardest to track. Once he appeared, Kid Buu wreaked havoc upon the universe before ultimately being destroyed by Goku. Surprisingly, this manifestation of pure evil has an unlikely connection with the former Supreme Kai, otherwise known as Grand Supreme Kai. Despite being killed at the end of Dragon Ball Z, Kid Buu is still impacting the series all the way into Dragon Ball Super's manga.
When Kid Buu first appeared millions of years ago, he fought against the various Kai who served during that era. After absorbing the hulking South Kai, he encountered Grand Supreme Kai, whose god power was still not enough to defeat the pink villain through traditional means. When Kid Buu successfully absorbed GSK, he turned into the fat Majin Buu our heroes have come to know and love, alternatively referred to as Good Buu.
Grand Supreme Kai getting absorbed turned out to be the best possible outcome short of actually defeating Buu, as his benevolent nature subdued the villain. Leashing the previously unrestrained chaotic evil, GSK's existence as part of Majin Buu weakened the monster. In a signature confusing plot twist, the evil remaining in Good Majin Buu eventually manifested into the aptly named Evil Buu, who re-absorbed the Good Buu and became the host body that would eventually revert to Kid Buu once more.
When Vegeta removed Good Buu from inside Evil Buu, there officially became two Majin Buu. The Super manga has confirmed that Grand Supreme Kai still exists inside of Good Buu. Free of the evil that used to inhabit Good Buu, GSK can even "switch places" with Buu, regaining his original appearance and control of their body for a short time. The Super manga reveals that even before Kid Buu appeared millions of years ago, GSK fought Moro, an ancient, evil wizard.
However, Kid Buu was killed by Goku at the end of DBZ, so how could he still have a relevant connection to Grand Supreme Kai in the Super manga? When GSK attempts to seal Moro the same way he did in the past, he realizes that he has lost his god power. This brings him to the conclusion that when Kid Buu and Good Buu split, Kid Buu managed to keep GSK's god power. That Kid Buu held god power is a surprise to Goku, although other than the villain's ability to instantly travel from planet to planet as a Kai would, he was apparently unable to make any further use of GSK's power.
Fortunately for our heroes, everything comes full circle in the final battle against Moro thanks to Goku's plea for Kid Buu to come back as a better person at the end of DBZ. In need of more god power, Grand Supreme Kai tracks down Uub, the human reincarnation of Kid Buu, and helps him unlock his latent god power to help Goku. It's unclear if GSK was unable or unwilling to reclaim his lost god power from Uub, or even if there's any of it left after using it to assist Goku.
Kid Buu and Grand Supreme Kai's fates appear to be forever entwined thanks to the monster's absorption of the Kai. Two halves of a whole, even with Kid Buu now reincarnated as an Earthling, the power unleashed with their reunion in the Super manga was the deciding factor in the universe-saving battle against Moro. The evilest monster to ever torment Universe 7 and the former Supreme Kai are the most unlikely of pairings, but their fateful encounter eons ago has shaped (and saved) the entire universe as our heroes know it.
