In Judaic and Islamic traditions, Sariel is the name of an archangel. However, some Judaic-Christian texts claim that Sariel is an Angel of Death, alongside Michael, Gabriel, and Samael. In other texts, Sariel is referred to as a fallen angel. Sariel is always depicted as being male, but the sprite is ambiguous enough to belong to either gender.
In Coptic tradition, Sariel is thought to be the angel of the church of Sardis, referred to in The Revelation to John, and has an immense knowledge of magic, magic rituals, and magic powers that give him the ability to lead others astray. Fittingly, his/her second theme is called "The Civilization of Magic".
Sariel's background changes throughout the battle: from a starry night, to a river with two floating yin-yang orbs, to what appears to be an inferno with human arms reaching out of the ground, and a collection of orbs showing Reimu and her shrine at another point in time.