The Voice of God. But not the voice of God. An entity in its own right. Rather like a Presidential spokesman. Speaks to Aziraphale in Good Omens. But is kept in the dark by Higher Authority concerning the true nature of the Ineffable Design all the same. When he assumes a physical form he appears as a young man clothed in golden fire.
In Cabbalistic theory (Judiac mysticism) the Metatron is the "bridge", the "gate", an intermediary between the Godhead and his human creation.
Christianity takes the logic of this a step further, identifying the Metatron with the Second Person of the Holy Trinity (who of course, in Christian theology, is God made man, a living bridge between the Godhead and humanity, who we know better as Jesus Christ.). Perhaps, with one eye on the US market, TP and NG would have had to disguise any direct appearance in this book by Jesus Christ lest they really be accused of blasphemy. Even though the return of Christ in glory is, of course, one of the culminating events of the Apocalypse (which in Good Omens is switched off by the principal characters before it gets to this stage, thankfully).