The start of Dragon Ball Z revealed the universe was ruled by Kais, deities of mounting cosmic power that each ruled different jurisdictions of the Dragon Ball Multiverse. The Kai in charge of the northern quadrant of the main setting's Universe 7 was referred to by Goku and the Z Fighters as King Kai, a jolly demigod that loved good jokes and was an unassuming martial arts master. In a shocking twist, King Kai was killed halfway during DBZ, but able to retain control of his body in the afterlife. And although the Dragon Balls have the power to restore life to even fallen gods, King Kai is still dead throughout Dragon Ball Super, much to his visible chagrin.
Following Goku's sacrifice to kill his evil brother Raditz, Kami guides him to the afterlife, where he is allowed to keep his body and instructed to run down Snake Way to reach King Kai. After a journey spanning several months, Goku finally arrives at King Kai's planet, discovering it's five times heavier than Earth's gravity, making him significantly stronger while the friendly Kai teaches him the Kaio-ken technique and the Spirit Bomb. Even after Goku was resurrected by the Dragon Balls and returned to Earth, King Kai periodically gave him and the Z Fighters advice telepathically and briefly trained several Z Fighters killed by the Saiyans.
During the Cell Games, as Super Saiyan 2 Gohan overpowered an increasingly frustrated Perfect Cell, the villain decided he would blow himself up, destroying the Earth and taking the Z Fighters with him. Quickly running out of options, Goku used Instant Transmission to teleport himself and Cell to King Kai's planet, reasoning that less innocent people would be caught in the blast. Goku, King Kai and his companions Bubbles and Gregory were killed in the explosion before Cell reformed and returned to Earth. With all of the deceased heroes able to keep their bodies, they immediately reappear in the afterlife -- complete with halos -- to help guide Gohan into destroying Cell for good.
King Kai takes his death in relatively good spirits, given the circumstances, as there is no immediate change in his role as a god. King Kai is more annoyed that Goku's sacrifice play had cost him his home than his actual death, especially for over a planet as seemingly insignificant as Earth. When reunited with the other three Kais that ruled over Universe 7's other quadrants, the North Kai is visibly embarrassed as the other Kais point out they have never actually seen a dead Kai before. By the start of Dragon Ball Super, King Kai rebuilt his planet but was frustrated that, in all the times the Z Fighters used the Dragon Balls, they never took the time to resurrect him and his companions. This would become a recurring joke through the anime series, with Goku always forgetting or stopping just short of resurrecting his former teacher.
Virtually every character in Dragon Ball has been dead at one point or another over the course of the franchise -- some on multiple occasions! One character that has been dead for quite some time and yet never received the benefit of a Dragon Ball-powered resurrection, even after asking, is King Kai. While his divine work remains unimpeded, King Kai has been longing for a new lease on life, if only to save face around the other Kais for the humiliation of being killed to save a single planet.
