Joan

Joan is the best friend of Ellen Leigh and one of the leaders of King Paimon Cult, along with Ellen.

Hereditary
At a funeral for her deceased mother Ellen, Annie denotes that there were several people present at her mothers' funeral that she didn't recognize. During her time at a support group, Annie reveals that she and her family had a long history of mental illnesses.

Grief-stricken at the death of her daughter Charlie, Annie meets Joan at her support group and Joan suggests that she perform a séance to communicate with her deceased daughter, claiming that she engages in them in order to speak with her deceased son. The séance appears to work, but it also causes more supernatural events to befall the family. Suspecting that her daughter's soul had become corrupted, Annie goes to Joan's apartment for advice only to see it vacant. Whilst going through her late mother's belongings, Annie discovers a photograph of her mother and Joan, revealing that they had an extensive history with each other. In the book, she finds a highlighted page dedicated to a demon known as Paimon, who desires a vessel of a young man.

In the end, the whole family ends up dead and Peter becomes possessed. He walks up to the family's treehouse to find cult members devoted to Paimon. Joan is present, revealing that she was the second-in-command for Ellen's cult and that she and the cult orchestrated all of the hauntings - including Charlie's death - as a means of summoning their dark god. It is therefore revealed that Joan had deceived Annie over having lost both her son and her grandson, given how the spirit she conjured in their seance was, in fact, King Paimon, masquerading as a human spirit.