Lillian is a twenty-eight-year-old woman who feels stuck in a life of monotony after taking the fall for her wealthy roommate during high school. She relies on a tough exterior and a sarcastic sense of humor to cope with her deep-seated insecurities. When she accepts a mysterious caretaking position at an expansive Tennessee estate, she finds a renewed sense of purpose and relates to the feeling of being an outsider.

Madison is a stunning, highly ambitious woman married to a prominent United States senator. Since her teenage years at the Iron Mountain Girls Preparatory School, she has ruthlessly pursued a life of extreme political power and immense wealth. She presents a perfectly curated public image of grace and civility, though beneath the surface she is exceptionally calculating and willing to manipulate situations to protect her family's status.

Bessie is the ten-year-old daughter of Jasper and his late first wife. She suffers from a mysterious condition that causes her to spontaneously combust when overwhelmed by intense emotions. Deeply protective of her brother, she assumes an adult level of responsibility to shield them both from further trauma. She remains guarded and hyper-observant, fiercely suspicious of new people and desperate to avoid drawing unwanted attention.

Roland is the ten-year-old son of Jasper and Jane, sharing his twin sister's unusual, fiery affliction. Unlike Bessie, he maintains a more childlike innocence and is eager for affection and comfort. He often follows his sister's lead but quickly warms to any sign of genuine kindness, showing a softer, less cynical approach to the strange circumstances of his life.

Jasper is a prominent United States senator from Tennessee who is currently undergoing a rigorous vetting process to become the next secretary of state. An immensely wealthy and handsome man, he approaches his family life as a purely artificial extension of his political campaign. He is deeply concerned with his public image and views any personal complications as threats to his ambition.

Carl is a former investigative reporter who now acts as Jasper's discreet fixer and logistical manager. Lean and intense, he is fiercely loyal to his employers and views his responsibilities through a strictly pragmatic, risk-management lens. He is tasked with keeping the twins' condition a secret, approaching the children as potential disasters to be managed rather than people to be comforted.

Timothy is the three-year-old son of Jasper and Madison. He possesses his mother's striking beauty but exhibits eccentric, almost robotic behavior. He gives his stuffed animals formal human names and spends much of his time quietly observing the activities around the estate, appearing isolated within the massive, pristine mansion.

Mary is a dedicated member of the household staff at the Roberts mansion. Professional and observant, she manages the family's meals and quietly works around the eccentricities of the household. She is deeply familiar with the family's secrets but maintains a discreet, unbothered demeanor regarding the unusual events on the estate.

Jane is Jasper's late first wife and the mother of Bessie and Roland. Though she passes away before Lillian's arrival, her legacy heavily influences the present story. She struggled significantly with the dissolution of her marriage and the stress of her children's unusual condition, keeping them isolated and adopting harsh methods to manage their fires.

Dr. Cannon is an eccentric, retired family physician whom Jasper trusts to evaluate the twins. He proudly claims expertise in both medicine and the paranormal, offering bizarre theories about the children being divine vessels or demonic entities. His strange medical theories provide a strong contrast to the children's actual, grounded needs.

Lillian's mother is a deeply apathetic and resentful woman who views her daughter as a burden. Living in a cramped house in a Tennessee valley, she has always prioritized her own interests over Lillian's well-being. Her casual cruelty and complete lack of parental support are the foundational sources of Lillian's deep-seated insecurities.

Mr. Billings is Madison's immensely wealthy and influential father. He is a pragmatic man accustomed to using his financial power to mold the world to his liking and protect his family's reputation. He treats problems purely as transactions, demonstrating the ruthless privilege that Madison later inherits.

Mr. Cunningham is Jane's father and the former patriarch of a powerful political family. His reputation was destroyed after he bankrupted his family while defending himself against charges related to a Ponzi scheme. He treats his grandchildren as a financial burden, eager to hand them over in exchange for a cashier's check.