Asariel is one of Metatron's most loyal angels who was recruited by Gadreel.
Asariel and Purah open the portal to Castiel and Gadreel.
Background
Along with Purah, Asariel guards the portal to Heaven, which was in a playground, using a woman as a vessel. In order for Castiel and Gadreel to gain access to Heaven, Castiel, having gained knowledge of pop culture, has Gadreel take him "prisoner." However, Asariel and Purah tell Gadreel they have orders from Metatron to keep the door closed to all Angels, on Heaven and on Earth. Gadreel refutes them by telling them it was Metatron's order to capture and bring Castiel to Heaven for questioning. Asariel and Purah begrudgingly agree and go about drawing the spell in a sandbox, and allow Castiel and Gadreel passage to Heaven.