Death

Rio Vidal is Death, who walks among the living as a witch and, at times, an FBI agent. She and Agatha Harkness were lovers, until Death came for Agatha's son Nicholas. She walked the Witches' Road as Agatha's coven's green witch, and came to collect Billy Maximoff for living twice. Agatha went with her instead.

First turns up in Agatha All Along.