Also called Gaia's children, they were created by Gaia in order to protect her, her Prophets and the Earth.

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They are regarded as the apexes of creation due to the sheer amount of power they were born with. Once one is dead or absent from the world, Gaia creates or assigns another Guardian. 

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In Park Taejin's case, she was described as being selected as the new "Jecheondaeseong", which is Mori's title. Since Gaia was the one to assign her as such, it's likely that she meant to replace Mori as the new guardian in his absence.

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Known Guardians:

• 1st Guardian, Ogre (deceased)

• 2nd Guardian, Jin Mori (ascended to a higher plane of existence, therefore considered absent)

• 3rd Guardian, Park Taejin (current)

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Given that by the end of the story it is now the "age of humans", it is likely because of this that Gaia decided to assign a capable human, Park Taejin, as the next Guardian rather than create another demon, but this remains a theory.