Demons are supernatural beings that exist according to a variety of religious traditions. In some traditions, they can be summoned for assistance; however, in contemporary Christianity, they’re typically viewed as evil. In Christianity, demons reside in Hell, which is ruled by Satan. Demons are assigned different ranks in a hierarchy, serving as “dukes,” “princes,” and the like, and presiding over other demons. Although they reside in Hell, demons are believed to travel to Earth and interfere with humans by either influencing them or possessing them.

The specific demon who possesses Gretchen in My Best Friend’s Exorcism is Andras, who appears in various religious texts about demons, such as The Lesser Key of Solomon, which was anonymously authored in the 17th century but likely compiled from and inspired by earlier texts. In this novel and other texts, Andras is a marquis of Hell, a high-ranking demon who commands several legions of other demons. He’s associated with owls and sometimes depicted as owl-like; he’s interested in violence, turning people against each other.

Different religious traditions have different stances on demonic possession, its stages, and whether or not it’s real. However, most traditions and the novel depict possession as happening progressively.