Mouse is a thirty-something freelance editor seeking an escape from her life in the city after a bad breakup. She agrees to clear out the dangerously cluttered home of her late, abusive grandmother, motivated entirely by loyalty to her aging father. Grounded and highly practical, she attempts to apply logic to the bizarre, unnatural phenomena occurring in the rural woods. When these rational explanations fail, she relies on her stubbornness and fierce dedication to her dog, Bongo, to survive.

Frederick Cotgrave is Mouse's late step-grandfather, a quiet World War II veteran who lived in terror of his malicious wife. Relegated to a single room in the cluttered house, he spent his time walking in the nearby woods and recording his observations. His hidden journal and a typed transcription of an old Welsh manuscript serve as Mouse's only warnings about the hidden terrors waiting outside.

A cruel and vicious woman whose malicious presence outlasts her death. Her obsessive collecting turned the house into an impassable fire hazard packed with trash, appliances, and eerie baby dolls. Her everyday domestic wickedness alienated her family and tormented her husband, leaving behind a physical and psychological mess that Mouse must clean.

A tall, pragmatic woman living at the neighboring commune who dresses in mismatched, colorful clothing. Rather than dismissing Mouse's fears, she openly accepts the existence of local supernatural threats. She pairs grounded practicality with an understanding of folk traditions, arming herself with protective hickory beads and a handgun.

A preternaturally pale and tall woman who confronts Mouse in the hidden world beneath the hill. She claims descent from a hidden lineage and presents an aura of otherworldly authority, yet she also appears desperate to end her long isolation. Her true motivations remain ambiguous as she guides Mouse deeper into the subterranean world.

Mouse's redbone coonhound, a simple animal driven entirely by his nose and his appetite. His keen animal instincts allow him to perceive unnatural dangers before humans do, though his tendency to bark at ordinary things occasionally causes confusion. Mouse's deep attachment to him anchors her to the property during the frightening events.

A friendly, physically strong young man residing at the commune across the road. He checks on Mouse shortly after she arrives and helps her move heavy objects out of the hoard. He provides a supportive, mundane anchor for Mouse and earnestly warns her to be careful in the woods.

A quiet, artistic potter who shares a home with Foxy and Tomas. Possessing a dry sense of humor, he offers Mouse a welcoming environment when she visits the commune for dinner, further cementing her connection to the supportive local community.

A local police officer in the town of Pondsboro. He listens patiently to Mouse's bizarre reports and hikes into the woods to investigate her claims, though he ultimately defaults to mundane explanations like teenage pranksters.

A goth barista working at the coffee shop in Pondsboro. She confirms the awful reputation of Mouse's grandmother and points Mouse toward local law enforcement resources when she becomes frightened by the strange occurrences.

An old friend of Frederick Cotgrave. He gave Cotgrave the mysterious Green Book diary, believing it represented a dangerous pursuit of forbidden knowledge that defied the natural order.

An ancient, extremely pale man who lives in the subterranean city alongside Anna. He enforces the hidden world's rules and threatens Mouse and Foxy when they trespass into their territory.

Mouse's aging father, who finds himself completely unable to face the emotional and physical burden of his late mother's house. He remains physically absent from the property, checking in on his daughter periodically via phone calls.