Remiel (レミエル?), also known as Ramiel (ラミエル?) is a demon in the series.
An angel of whirlwinds, thunder, hope, and earthquakes who has various apocalyptic functions. He is charged with leading souls to judgment and is often called the angel of "true visions." He is believed to be a fallen Watcher as Ramiel in the book of Enoch. While he is an Archangel in Christian and Islamic lore as Remiel.
Ramiel means "Thunder of God" and Remiel means "Mercy of God" or "Compassion of God".
Remiel is fought in Amnesia's Ahlabot on the Neutral, Light-Law, and Dark-Chaos routes.
Devil Children Fire/Ice Book
Remiel is one of the angel servants of Lord Enoch. He is summoned by Lena to confront Jin when he gets in her way. Upon his defeat he will flee in order to fight Jin another day. After the fake Enoch's death he will try to search for Lena in Niflheim, where he can stumble across Jin and ask to join him, as he learned that the Enoch he was serving was in fact a fake after his defeat and wishes to save Lena.
Remiel appears as the entity possessing Amane, working under Metatron's orders. During the 5th and 6th day, he speaks to the protagonist (through Amane) proposing that he becomes a new Messiah as the King of Bel, and is revealed to be suppressing the demon Jezebel, who is sleeping within Amane's body. On Day 7 of Amane or Gin's routes, Remiel will enable the party access to Amane's soul in order to defeat and absorb Jezebel. Upon eliminating her, Remiel leaves Amane's body, entrusting her with guiding the party through the rest of the day.
Remiel will be fought early in Naoya/Kaido's route, while in Amane's body. In Yuzu's route, he is fought along with Izuna, Sariel, and Anael in the last battle. Defeating him in either of these routes unlocks him for fusion.
On the 8th Day of Naoya's route it is mentioned he is an observer angel and his disappearance during the 8th Days means he has returned to heaven to prepare God's Thunder, which can incinerate the world and demons who might try to enter Heaven.
Devil Survivor 2
"One of the seven archangels serving God. His duty is to spread their instructions. The words he conveys appear as visions to people. On the day of judgement, he knows which souls will be saved and which will be destroyed. His duty is to guide the souls that will be saved."