January is a 17-year-old girl with "coppery red" skin and frizzy hair. She lives a pampered but highly controlled life in Vermont as the ward of wealthy Cornelius Locke. Longing for adventure but desperate to earn her guardian's approval, she suppresses her wild nature to behave as a proper, obedient young woman. Her sheltered existence changes when she discovers a mysterious book that hints at other worlds and awakens her dormant curiosity.

Locke is the incredibly wealthy head of the New England Archaeological Society and owner of W.C. Locke & Co. He provides January with an elite, luxurious upbringing but demands absolute propriety and compliance in return. An avid collector of rare objects from across the globe, he prizes order, stability, and rationality above all else, showing zero tolerance for fantasy or disobedience.

January's father is a dark-skinned, heavily tattooed man who works tirelessly collecting rare artifacts for Mr. Locke. His constant expeditions keep him separated from his daughter, creating a lingering sense of guilt and sorrow. He possesses a profound appreciation for language, forgotten myths, and the cultural artifacts he encounters during his journeys.

A wheat-haired, freckled young woman featured in the mysterious text January reads. Raised in poverty by her aunts and grandmother on a farm in Ninley, Kentucky, "Ade" possesses an irrepressible, restless spirit. Her brief, life-altering encounter with a boy from a world of sea and stone sparks a lifelong obsession with discovering hidden entryways to other places.

Jane is a dark-skinned woman with callused hands, an unusual accent, and a quiet but unshakeable confidence. Brought in at Julian's request, she initially keeps an emotional distance from January. Over time, her background as a capable survivor becomes evident as she steps up to defend her young charge from hostile forces.

Samuel is the handsome, olive-skinned boy who lives next door to the Locke estate and works for his family's grocery business. He is January's only true childhood friend, sharing her secret love for adventure tales. As he grows older, he remains a steadfast, loyal presence who sees past January's obedient facade.

A pale, sneering member of the New England Archaeological Society. He exhibits a frighteningly cold aura and uses his position of power to intimidate those he considers inferior. He specifically targets people with darker skin, viewing them as objects rather than humans.

An unsettling associate of Mr. Locke who works on behalf of the Society. He wears a specialized mask and uses a unique compass to track people, acting as a dangerous enforcer who eliminates threats to the Society's preferred world order.

An older resident of Arcadia, affectionately known as "Sol." He acts as a guard for the community of runaways, armed and cautious about outsiders until he hears familiar names.

The chieftainess of Arcadia, a hidden world composed entirely of runaways and outcasts. She is a practical, fair-minded leader who ensures everyone in her diverse community pulls their weight to help the society survive in a harsh environment.

Adelaide Larson's aunt, who still lives on the family's farm in Ninley, Kentucky. She is a nostalgic woman who has spent decades hoping her adventurous niece will return, regretting the family's decision to sell their hayfield to a strange buyer.

January's first nursemaid at Locke House. She is a strict disciplinarian who enforces Mr. Locke's demands for absolute propriety. She eventually resigns from her post due to her intense frustration with January's pet dog.

January's fiercely loyal dog, known simply as "Bad." Originally forbidden from the estate, he becomes a permanent resident after January writes a story about keeping him. He is highly protective of his owner, willingly attacking anyone who threatens her.

A nurse working at the Battleboro asylum where January is confined. While she enforces the strict and oppressive rules of the institution, she is capable of small mercies, taking pity on a terrified patient.

A woman residing in the Written. She honors the traditions of her world by permanently marking major life events onto the skin of her family members.

The matriarch of the Larson family in Kentucky. She helps raise her granddaughter in a busy, crowded farmhouse before the young girl sets out to find adventure.