Charlie Graham is an odd duck of a girl whose anguish comes through in her blankly haunted features. Her body holds the soul of King Paimon, who has taken over since she was a baby.
While inhabiting the body of Charlie, King Paimon is shown to be confused and disconnected from reality. Though he has a cult of devout, lifelong followers, he himself is not shown to seek a life on Earth. In actuality, he is perpetually out-of-place and scared from what he is witnessing through Charlie's eyes. According to Alex Wolff, who portrays Peter, Paimon's mortal ineptitude is interpreted as mental illness by the world.
At the funeral home, Charlie receives weird looks from a stranger and draws on her sketch book during Annie's eulogy. After the funeral, Charlie continues to receive weird looks from other strangers. At school, a bird flies into a window, killing it in the process. Charlie steals scissors and cuts the head off of the bird before using it as an art project after school. She has done this with other animals' heads and keeps them in a box as arts and crafts projects.
To attend a party, Peter lies that he is going to a school event, and Annie forces him to take Charlie with him to ensure that Peter won't do anything irresponsible. Unsupervised, Charlie eats cake containing nuts, which she is allergic to, and falls into anaphylactic shock. As Peter drives her to a hospital, Charlie leans out of the window for air. Peter swerves to avoid a dead deer and Charlie is decapitated by a telephone pole. In shock, Peter silently drives home and leaves his sister's corpse in the car for Annie to discover the next morning. The family grieves following Charlie's funeral, heightening tensions between Annie and Peter. Peter is plagued by Charlie's presence around the house.
At the end of the film, Peter kills himself by jumping out of a window, making Paimon enter and pertinently posses him. As he goes into the treehouse, fellow cult member Joan confirms that Charlie was actually Paimon and her body was a temporary host. The real soul of Charlie has apparently disappeared years ago and was possibly freed when she finally died.