Cora is a recently widowed woman with a burgeoning passion for the natural sciences. Finding herself free from a restrictive and unhappy marriage, she leaves London for the Essex countryside to investigate rumors of a mythical sea creature. She is fiercely independent, intellectually curious, and determined to live life on her own terms.

Will is the local vicar of Aldwinter, a small Essex village plagued by rumors of a monstrous serpent. He represents reason and institutional faith, working to dissuade his congregation from their superstitious beliefs. He is an intelligent, deeply thoughtful man who finds his quiet, predictable life disrupted by outside arrivals.

Luke is an ambitious and brilliant London surgeon, sometimes nicknamed the Imp due to his diminutive stature. He possesses a forward-thinking medical mind and desires to advance his profession through experimental procedures. Despite his professional confidence, he struggles to express his deep romantic feelings for Cora.

Stella is Will's gentle and devoted wife. She embodies the traditional role of a village vicar's partner, finding deep contentment in her home and her three surviving children. She frequently suffers from lingering illnesses but maintains a positive, quietly courageous outlook on life.

Francis is Cora's uniquely detached and intelligent son. He interacts with the world on his own terms, expressing his emotions by collecting eclectic objects he calls his treasures. His unconventional behavior frequently isolates him from other children.

Martha works as Francis' nanny and serves as Cora's closest confidante. She is a fiercely dedicated socialist who actively campaigns for better housing conditions in London. Though she dislikes the rural Essex countryside, her loyalty to Cora keeps her close.

George is a privileged, wealthy doctor and Luke Garrett's closest friend. He often subtly provides financial assistance to Luke without bruising his pride. His interactions with Martha soon spark a developing interest in political activism and social housing.

Charles is a brash, friendly, and wealthy man who holds suspicious views regarding evolution. He serves as the primary connection between Cora's London circle and the rural village of Aldwinter.

Katherine is Charles' socially connected wife. She facilitates Cora's introduction to the Ransome family by recommending that her husband write a letter to Will.

Joanna is the eldest daughter of Will and Stella Ransome. She possesses a theatrical imagination and participates in local superstitions regarding the Essex Serpent alongside her friends.

Naomi is a local Aldwinter girl and Joanna's close friend. She feels deeply affected by the rumors of the monster and participates in attempts to ward it off.

Henry is a local bargeman who makes his living on the Blackwater estuary. He relies on the water for his livelihood and harbors a quiet wariness about the unusual occurrences in the area.

Cracknell is an eccentric, grieving man who lives at World's End, the final house before the marshes. Having recently lost several family members, he firmly believes the Essex Serpent is real and is responsible for the disappearance of his livestock.

Michael is Cora's late husband. He was a controlling civil servant who restricted Cora's intellectual pursuits and personal freedom during their marriage.

Edward is a man who becomes the subject of an experimental, life-saving surgery. He carries guilt over his past behavior, acknowledging that he previously bullied his attacker.

Samuel is a man pushed to violence after enduring repeated bullying. He retaliates by stabbing his tormentor.

Thomas is a disabled beggar who acts as an unofficial tour guide for Colchester's earthquake ruins. He shares local legends about the Essex Serpent with visitors.

James is one of Will and Stella's young sons, growing up in the quiet village of Aldwinter.

John is the youngest son of Will and Stella. He joins his older sister in superstitious games to ward off the local monster.