Olivia is a teenage orphan who communicates through writing and sign language due to a speech disability. She lives an isolated life at the Merilance School, finding solace only in a cryptic green journal left by her deceased mother. When she receives a letter summoning her to her family's estate, Gallant, she discovers she has the ability to see ghouls. She sets out to uncover the truth behind her family's cursed history, driven by a deep curiosity to explore the hidden domains beyond the estate's garden wall.

Matthew is Olivia's cousin and the current resident of Gallant. Burdened by the tragic loss of his immediate family, he aggressively demands that Olivia leave the estate upon her arrival, viewing her presence as a complication to his duties. He suffers from severe night terrors that require his caretakers to bind his wrists while he sleeps. Though he struggles with a learning disability, he finds peace in playing the piano and views his duty to protect the estate as a grim, solitary obligation.

The master is a skeletal, inhuman entity who rules the dead, mirrored version of Gallant. With skin like cold ashes and milky white eyes, he commands shadow guards and is intensely focused on acquiring missing pieces of bone to make himself whole. He orchestrates events from the other side of the garden wall to trap members of the Prior family. He possesses the unique ability to project his voice, which he uses to terrify and manipulate others.

Grace is Olivia's deceased mother. She originally lived at Gallant with her brother Arthur before fleeing the estate shortly before Olivia's birth. She leaves behind an illustrated journal containing cryptic warnings about the property and her experiences beyond the garden wall. Her spirit lingers as a ghoul wandering the halls, offering silent clues to her daughter about the true nature of their home.

Hannah is an older woman who works as a dedicated caretaker at Gallant. She serves as a protective, maternal figure for the surviving Prior children, often lying to keep them out of harm's way. She initially tries to shield Olivia from the estate's dangers by insisting the girl stay indoors after dark. Having witnessed the erratic behavior of previous generations firsthand, she offers Olivia essential context about her mother's past.

Edgar is Gallant's other dedicated caretaker. He is proficient in sign language, immediately establishing a vital line of communication with Olivia when she first arrives at the estate. He handles the physical maintenance of the property and physically restrains Matthew during the boy's severe night terrors. He works closely with Hannah to maintain a sense of normalcy in a profoundly unnatural household.

Arthur is Olivia's uncle and Matthew's father. Although he has been dead for nearly a year, his name appears on the letter that brings Olivia to Gallant. During his life, he suffered greatly from the family's curse, ultimately taking his own life to escape the ceaseless nightmares. His ghoul remains trapped at the estate with half his face missing.

Thomas is Matthew's younger brother. Years ago, he disappeared after being lured across the garden wall through his dreams. His absence deeply wounds Matthew and serves as a constant, painful reminder of the threats lurking outside the house. He represents the severe physical and emotional toll the shadow world exacts on the Prior bloodline.