Indeed, the Guardians of the Universe were the ultimate line of defense for the original positive matter universe. Having originated on the planet Malthus, these immortal beings migrated to planet Oa in the center of their reality to oversee and protect it. Their counterparts on Earth-Two did not, as the Buddak were intent on harnessing their red version of the Guardian's emerald energy for evil. While some of the Guardians migrated from Malthus to Earth-One to become leprechauns such as Jack O'Lantern's pal Fergus, since there was no Oa in Earth-Two's universe, a larger proportion of their dopplegangers journeyed to that world such as Moon O'Day who met the golden age Wonder Woman, and fellow leprechauns such as Shaggy, Hoppy and Woggle as well as Owen Cooley who was a pest to Earth-Two's Green Lantern and a counterpart to former Guardian turned friend of Earth-One's Green Lantern in "Old Timer" aka Appa Ali Apsa.
However, there were some who took the place of the elite Malthusians who volunteers to serve infinity on Oa, in the form of a Chinese Embassy on Earth-Two that had operated throughout the centuries. Green Lantern had initially encountered them when he was given the power battery under undocumented circumstances. In 1945, one of their number contacted the hero's alter ego Alan Scott through the lantern to warn him of Solomon Grundy's return.
Then in 1946, Green Lantern and his pal Doiby were brought back to the medieval past of Camelot to protect it from a viable threat, and ended up meeting the protectors of the Green Flame's power battery who sent the pair back to their own era. Although not as regular in their contact with their apprentice as were the Guardians with Hal, Alan definitely appreciated them.