Bearer of Keys
Bearer of Keys is a respected Celestial Spirit mage who lives in the Celestial Spirit Realm and is entrusted all 12 of the golden Zodiac keys including the 13th.
They hold the keys and protect them from the wrong hands, and from being used as vessels of great power.
At a high enough power that is built from fulfilling their duties as Bearer of Keys and train hard to hold greater amounts of power like meditation, they themselves could create more of the celestial spirit keys, and drop them down into Earthland for others of the same magic.
And if they possess great enough power, they may summon the most powerful of all the Zodiac Keys combined, the Celestial Spirit King and not breaking one of the Zodiac Keys to summon him, instead using themselves as a gate, like making yourself a key so the King can pass through.
Though Bearer of Keys are more powerful than regular Celestial Spirit mages, they are bound by the Spirit Realm and can be used as a gate into the Spirit Realm. If taken into the wrong hands, their body will rip apart as the gate opens to the Realm inside of them, or can be used to destroy the Spirit Realm if their body does not rip apart.
More than that though, the Bearer of Keys can also transfer magic from the Spirit Realm and be used as a weapon. Either against their will or not, if they are caught into the wrong hands, their life will cease.