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Metatron is the archangel whom according to Castiel, wrote down the Word of God when creation was being formed. The Word of God was inscribed on multiple stone tablets in a ancient language that most beings, including angels, can not read. One of these tablets containing the Word had instructions on how to kill the Leviathans should they escape from Purgatory. Metatron was later mentioned by Kevin Tran when reading the Word of God to Crowley.
While Kevin was reading the tablet on demons, the prophet saw a personal note by Metatron. It mentions the archangel taking leave of his Master and letting humans take responsibility of the compendium of tablets.
His current status and location is unknown, largely because of his departure. Why he did this is unclear. It is also not yet mentioned how he is related to the other four archangels.