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. 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.
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. After Piccolo is defeated in the World Martial Arts Tournament, relative peace remains for five years. Kami proposes Goku to become the new Guardian of Earth, but Goku refuses before leaving with his future wife Chi-Chi. The arrival of the Saiyans sparks an entirely new conflict, in which Kami 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, a new Guardian claims the throne and creates new, more powerful Dragon Balls.
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).
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).
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.