Father Gregory Sargent
He has just been appointed to a new parish. It's his task to become well acquainted with his congregation and to gain their trust. This matter is greatly complicated when Susan's father comes to him asking for help with her strange condition. Over the course of the book, Sargent is forced to reconsider his convictions about evil, human nature, and the power of God. He doesn't conclude that the girl is possessed, but he doesn't dismiss the idea either. In the end he believes that each person harbors some evil within their soul which can cause them to do unspeakable things. One doesn't need to believe in demons in order to lose oneself to darkness.
Susan Garth
She's 16-years-old and a fairly normal kid until right around the time Sargent moves into town. Suddenly she becomes a different person, losing her mind. She stops attending church, vehemently refusing to go. She begins cursing, throwing fits, having seizures, and all around behaving erratically. When the priest becomes involved, she chooses him to be her preferred victim. At one point she's in a session with him when she strips naked and attacks him for no apparent reason. As the story progresses, its revealed that she's beaten by her dad. He's never been a safe person for her, and since he's a single parent, she's pretty much powerless. In the end Russel spares passing judgement on Susan, but he does hint that she's responsible for her actions regardless of spiritual questions.
Bishop Crimmings
Bishop Crimmings is a personal friend of Father Gregory who comes to visit and help him settle into his new position. As a senior leader in the church, he accompanies Sargent in many of his interactions with the girl. He does not, however, take charge of the case. He uses it as an opportunity to evaluate Sargent's capabilities. He's a man of reserve and immense wisdom who urges Sargent to reconsider some of his religious convictions.
Mr. Garth
Susan's father is a somewhat stubborn man. He takes her to church every Sunday until her fits begin, but he changes his tune afterwards. He begins listening to this friend of his who professes anti-Catholic sentiment obsessively, poisoning Garth's mind. Eventually readers learn that Garth punishes Susan with physical violence, which he considers perfectly appropriate. Although he is the one to first reach out to the priest for help, he ends up opposing him at every turn.
