Metatron was an angel and the Scribe of God who recorded The Word of God. In Heaven, Metatron was personally selected by God to write down His word and then God disappeared. Eventually, out of fear that the Archangels would steal the information of God from him, Metatron left Heaven and hid among native American tribes and modern day Americans until discovered by the Winchesters.
Learning that all the Archangels were either dead or in Hell, Metatron then sought to take vengeance upon the other Angels for forcing him to leave, using Castiel to make a spell writen on the Angel tablet, and emptied Heaven of all angels. Due to Metatron's actions caused many angel rebellions on Earth and upon his own disappointment of being alone in Heaven, Metatron started working to rebuild with Gadreel as his second in command, using the angel tablet to attain God-like power, accepting only the angels he sought fit and either killing or using those he didn't like. Once done so he attempted to become a deity to mankind itself, rallying many human followers until confronted by Dean himself. After Castiel shattered the angel tablet and tricked Metatron into revealing his true intentions, at the cost of Gadreel's self-sacrifice, Metatron was overthrown and locked in Heaven's dungeon, but not before he killed Dean and caused his transformation into a demon.
I did leave out some key parts so I would not spoil too much but I will say Metatron's character does play a big role in the show. From when he is first introduce Metatron is weaved in and out of the show!
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