Short answers, pulled from the story.
Metatron is an angel in Judaism, described in the Talmud and in mystical Kabbalistic texts as the celestial scribe who records the deeds of Israel. He is also known as the Prince of the World, the Prince of the Presence, and in 3 Enoch is called the Lesser YHWH. In early Kabbalah and rabbinic literature, Metatron is the name the biblical figure Enoch received after his transformation into an angel.
The precise origin of the name Metatron is unknown and there is no scholarly consensus. Proposed etymologies include the Latin word metator meaning guide or forerunner, the Greek words meta and thronos meaning one who serves behind the throne, the Hebrew verb memater meaning to guard or protect, and possibly the Old Persian name Mithra. Philip Alexander suggested it may originally have been a magical word, similar to Adiriron and Dapdapiron.
The Babylonian Talmud mentions Metatron by name in three places: Hagigah 15a, Sanhedrin 38b, and Avodah Zarah 3b. In Hagigah 15a, Elisha ben Abuyah sees Metatron seated in heaven and mistakes him for a second deity, prompting the rabbis to have Metatron struck sixty times with fiery rods to demonstrate that he is an angel and not a god. Sanhedrin 38b uses Exodus 23:21 to clarify that Metatron cannot pardon sins or serve as a messenger of forgiveness.
In Jewish apocrypha and rabbinic literature, Metatron is the name Enoch received after God transformed him into an angel. The basis is Genesis 5:24, which states that God took Enoch. 3 Enoch, the text where the name Metatron first appears, describes Enoch's flesh turning to flame and his being placed on a throne next to the throne of glory before receiving the name Metatron.
Metatron is attested in early Islamic writing by al-Kindi and al-Masudi. Al-Suyuti identified him as the angel of the veil who alone knows what lies beyond. Ahmad al-Buni described Metatron in magical texts as wearing a crown and a lance connected to the Staff of Moses. Muslim heresiologists used belief in Metatron as evidence that Jews venerated an angelic being rather than God, and Ibn Hazm recorded a tradition that Jews celebrated Metatron as a lesser god for ten days each year.
Alan Rickman played Metatron in the 1999 film Dogma. Derek Jacobi portrayed the character in the 2019 television adaptation of Good Omens. Alex Hassell played Metatron in the 2019 BBC and HBO adaptation of Philip Pullman's His Dark Materials. Curtis Armstrong played Metatron in the television series Supernatural, which premiered in 2005.