Despite their names, the Guardians' role is closer to that of an overseer, and they are not necessarily stronger than all those whom they oversee; while Kami demonstrated that he was likely the strongest being on Earth at the time of his introduction in the Piccolo Jr. Saga, he was quickly surpassed by Goku and many others soon after, but still remained the Guardian of Earth until the Imperfect Cell Saga, over a decade later. Guardians have an indirect reporting relationship to one of the Kais (in the case of the Guardian of Earth, this is King Kai). Another way of looking at this is that Guardians are more supervisors than they are managers. They have many of the powers associated with gods, including the ability to see and sense events around the world, to travel to and guide souls to Other World, and to be a general protector of the people. They also are long-lived, though not immortal. Unlike more traditional gods, the role of Guardians is an office, which can be vacated and subsequently refilled by a mortal. However, the average citizens of Earth are unaware of the existence of these Guardians, as demonstrated by Mr. Satan when he is puzzled by the role Dende plays. To become a Guardian, one must not have any evil in the heart, as the previous Guardian refused the Nameless Namekian for having his heart tainted by the corruptions of Earth. In Dragon Ball Super during the God of Destruction Beerus Saga, it is revealed that Guardians like Dende possess godly ki allowing them to sense the godly ki of other deities. From this it can be assumed that upon becoming a Guardian, one attains godly ki.
List of Guardians
Although Buddhism and Christianity exist on Earth in the Dragon Ball universe, most people are completely unaware of the existence of the true god, being the Guardian. Planet Earth has been overseen by Guardians for as long as the history given on the Dragon World dates back, and Mr. Popo has been the servant of several of them. The guardians preceding Kami are referred to as the Kamis of the Past in Daizenshuu 7, and Kami's former master is referred to as Kami of the Previous Generation. Additionally Daizenshuu 7 also reveals that Mr. Popo and Korin where sent from Other World to assist the planet's Guardian. Mr. Popo is also responsible for finding a new Guardian if the previous Guardian is unable to find a successor. The Nameless Namekian, as well as Garlic in movie/filler only, assumed the role of apprentice(s) to Earth's current Guardian in Age 431. Thirty years later in Age 461, the Nameless Namekian assumes the role and name of Kami, after splitting with his evil half in order to do so. This evil half would rename itself the Great Demon King Piccolo, and would go on to spark worldwide conflict. Since Kami and King Piccolo are one and the same, Kami was powerless to stop him due to the underlying fact that he would perish as well if he killed Piccolo. However, no other being on Earth was anywhere near powerful enough to halt Piccolo's rampage. Luckily, after years of conflict, a martial artist called Master Mutaito seals King Piccolo within an Electric Rice Cooker, trapping him inside for centuries. This allows Kami to survive and continue to serve as Guardian, creating Shenron and the Earth Dragon Balls to give the people of Earth hope for the future after the devastation caused by the conflict with King Piccolo's Demon Clan. However due their power to grant a single wish, many people sought the Dragon Balls for selfish reasons and unintentionally resulted in several conflicts, causing Kami to regret he created them.
Earth sees a time of peace, until King Piccolo is unleashed on the world again over three-hundred years later, only this time is stopped and killed by a new, far more powerful warrior on Earth, Goku. As he was dying, King Piccolo transferred his soul into an egg which ironically prevents Kami's death. Goku soon after meets and trains under Kami in order to stop his evil half’s reincarnation, Piccolo. Kami also revives Shenron who had been killed by King Piccolo despite having previously decided to not restore the Dragon Balls due to the conflicts they caused and having allowed King Piccolo to regain his youth, though Goku and his friends' noble actions as well as Goku agreeing to train under him convinced Kami to change his mind. Kami also permanently removes Goku's Tail in order to safely restore the Earth's Moon which was destroyed previously by Jackie Chun to end the threat posed by Goku's uncontrolled Great Ape form. After Piccolo is defeated in the World Martial Arts Tournament, Goku spares his life to have a rival and prevent Kami's death due to their Life Link, despite Kami's protests having decided to sacrifice his life due to feeling he wasn't worthy of the role due to his failures, but Master Roshi and Goku convince Kami that it is thanks to his creation of the Dragon Balls that lead Goku and his friends coming together in the first place, convincing Kami that he was indeed worthy of his position. Kami recognizing Goku's pure heart and strength proposes Goku to become the new Guardian of Earth, but Goku refuses before leaving to marry his fiancee, Chi-Chi. For the next five years, the Earth is relatively peaceful, though the arrival of the Saiyans sparks an entirely new conflict, in which Kami convinces King Yemma to allow the deceased Goku to train under King Kai after he sacrifices himself to kill his evil elder brother, Raditz and also trains Tien Shinhan, Yamcha, Krillin, Chiaotzu, and Yajirobe in order to combat the impending threat. Piccolo is later killed by Nappa in the battle, and thus Kami dies, leaving the Earth without a Guardian for around a month until revived by the Dragon Balls on Namek.
During the conflict with the Androids, Kami does little but offer support for the Dragon Team, however he does evidently learn of Cell's arrival, thus opts to merge with Piccolo into their original power under the Nameless Namekian. After doing so, Kami unites with Piccolo, leaving the Earth without a Guardian for some time. Shortly after, Goku enlists Dende, a young Namekian boy, to be Kami's successor, to which he accepts. Thus, after the defeat of Cell with Mr. Popo's approval, a new Guardian claims the throne and recreates the Earth Dragon Balls, making them more powerful than before. Due to the influence of Kami, Piccolo chooses to remain at The Lookout to assist Mr. Popo in mentoring the inexperienced Dende helping him grow into a proper Guardian.
Seven years later, during the conflict with Majin Buu, Dende is nearly killed by Super Buu, but is saved by Mr. Popo. Dende survives the remaining conflict, and even travels to the Sacred World of the Kai to watch Goku and Vegeta battle and defeat Kid Buu. After returning to the newly restored Earth, a time of peace ensues.
Despite his godly position, Dende joins his friends in various celebrations such as Bulma's birthday party, where he and his friends encounter God of Destruction Beerus and his attendant Whis during their search for the Super Saiyan God. Using his ability to sense Godly Ki, Dende is able to confirm when Goku obtains the Super Saiyan God form following the ritual with Vegeta, Trunks, Goten, Gohan, and Videl (pregnant with Pan). Dende also monitors Uub, the Earthling reincarnation of Kid Buu whom he requests Goku train in the future and is revealed to have been keeping an eye on the reformed Android 17 following the Cell Games. He assists Goku in finding Android 17 so he can recruit him into Team Universe 7 for the Tournament of Power which resulted in Android 17 winning preventing Universe 7 from being erased and lead to the revival of the other erased Universes due to his wish to Super Shenron which also increased the Mortal Level of Universe 7 due to the selfless nature of the wish.
In the Dark Empire Saga, Xeno Dende is the Guardian of Earth of the Xeno world main timeline who is captured by the Dark Empire and brainwashed to serve them by creating the Dark Dragon Balls. However he is freed after Demon Realm is sealed within the Time Labyrinth and transported to the Time Nest by Chronoa.
In Dragon Ball GT, Dende is now full-grown and allows Goku and his new trainee Uub to train at the Lookout. Dende provides support for the Z Fighters throughout the remainder of the series, as he is unable to do anything against the enormous powers of the villains that attack Earth. He is taken under the control of Baby, but is turned back to normal with the use of Sacred Water. After Goku defeats Omega Shenron and departs with Shenron, Dende returns to the Lookout and continues his role. Nothing beyond this is ever explored.
There is no Kami on Namek, for that is a term from Earth. It can be arguably assumed that the Guardian of Namekians is their Grand Elder Guru, who watches over his people much like Earth's Kami and who created a set of Dragon Balls for them. After Guru's death in the Frieza Saga, the Namek elder Moori is given the position and control of the Dragon Balls which recharge after 130 days (one Namekian year). In between the Frieza and Kid Buu Sagas, Moori increased the power of the Namekian Dragon Balls so that more than one person could be restored to life at once.
In the anime only, King Kai tells Goku that Planet Vegeta was destroyed by its own version of Kami via a meteor storm he summoned after realizing how evil the Saiyans were. Shortly afterward it is revealed, however, that Frieza was to blame for Planet Vegeta's destruction. It is clear that Frieza is not actually the Guardian of Planet Vegeta in the sense that Kami is of Earth and as such is likely that King Kai's story was fabricated to keep Goku from learning of Frieza and seeking him out (considering that King Kai later reveals that he is aware of the existence of Frieza, and begs Goku and others not to fight him because of his immense power). Additionally the story may have been inspired the fact Beerus ordered Frieza to destroy Planet Vegeta due to King Vegeta's stinginess and the evil of the Saiyans (though Frieza had his own reasons and Beerus' order was just one factor in his ultimate decision to destroy the planet and most of its population), which King Kai may have been aware of and used as inspiration for his Guardian of Planet Vegeta cover story.
The Konatsian Wizard is referred to as the Guardian of Planet Konats in Daizenshuu 6. This would fit with his essential role in ending the Hirudegarn war restoring peace to Planet Konats and may explain how he came into possession of the mystical artifacts such as Brave Sword, two Hero's Flutes, and the Music boxes used to seal Hirudegarn.
In Dr. Slump, there is a Kami of the Galaxy that exists in prehistoric times and is responsible for placing Gatchan's Egg on prehistoric Earth in an attempt to prevent further development of human civilization to keep them primitive and pure. Unfortunately the Egg was found by a time traveling Senbei Norimaki who brought it to Penguin Village, thwarting the Kami of the Galaxy's plan as Gatchan became an adopted member of the Norimaki Family.