Koh is a researcher with the Sons (of Saint Coinach) who, when exploring the newly unearthed Crystal Tower complex, came upon an orb of mysterious origin. Upon closer examination, she awoke the soul that slumbered therein - Noah, the famed Third Astral Era archmagus of Allag and the court-appointed instructor of Unei and Doga of the imperial family. In that moment, Noah's spirit escaped its crystalline gaol and flowed into Koh's body, taking possession of it as a temporary vessel.
Koh and Noah are as different as north and south. Noah displayed great passion toward both her research and apprentices, and was known to oft behave with wild abandon. Indeed, it appears that the imprisonment of her spirit was the result of an uncontrolled bout of simple curiosity.
Koh, by contrast, is a woman as timid as she is shy. When the archmagus returns control of her body - usually after an extended night of drink and debauchery - the Keeper of the Moon is in a constant state of fluster, owing to the excess attention Noah commands while she is in control.
(Referenced from Encyclopedia Eorzea, Page 200)