Cora is a recently widowed Londoner with a deep interest in natural sciences and botany. Freed from a restrictive and unhappy marriage, she embraces her new independence and relocates to the Essex countryside. She quickly becomes fascinated by the legend of the Essex Serpent, believing it to be an undiscovered prehistoric creature rather than a mythological monster.

Will is the local vicar of Aldwinter, a small and isolated Essex village. He is an intelligent, pragmatic clergyman who views the rumors of the Essex Serpent as godless superstition and fights to keep his congregation grounded in reality. He loves his family deeply, though his comfortable worldview is challenged when Cora arrives with her scientific theories.

A highly skilled and ambitious London surgeon nicknamed "the Imp" for his short stature. He is a forward-thinking medical professional who pushes the boundaries of experimental surgery. He secretly harbors a deep infatuation with Cora Seaborne, whom he met while treating her late husband.

Will's gentle and beautiful wife. She loves her family completely and finds deep contentment in her life in Aldwinter. Stella suffers from a lingering illness she brushes off as the flu, though she maintains a positive, welcoming demeanor toward outsiders.

Cora's fiercely loyal companion and Francis's nanny. Martha is a passionate socialist who cares deeply about fixing the social housing crisis in London. She dislikes the rural Essex setting but stays by Cora's side out of complete devotion.

Cora and Michael's introverted, eccentric son. Francis displays unusual behavior for a child his age, preferring isolation and collecting random objects he calls his "treasures." He uses these items to process his emotions and understand the world around him.

A wealthy, privileged doctor and Luke Garrett's closest friend. Spencer belongs to the bourgeoisie, which puts him at odds with Martha's socialist views. He is generous with his money and often funds Garrett's living expenses anonymously.

A wealthy, brash, and friendly associate of the Seabornes. He is involved in politics and serves as the primary connection between Cora and the Ransome family, introducing them via letter.

Charles's wife, a sociable and polite woman who helps facilitate Cora's introduction to the village of Aldwinter.

Will and Stella's daughter. She is an imaginative young girl who gets caught up in the local superstitions surrounding the Essex Serpent, much to her father's dismay.

A local Aldwinter girl and friend to Joanna. She is deeply afraid of the Essex Serpent and participates in rituals to keep it at bay.

A local bargeman in Aldwinter who works on the Blackwater estuary. He is wary of the water due to the local legends and keeps a close eye on his daughter.

An eccentric, grieving local man who lives at World's End, the last house before the estuary. Having lost multiple family members in recent years, he firmly believes the Serpent is real and acts as divine justice.

A man whose life is saved by Luke Garrett's experimental surgery. During his recovery, he admits to having bullied the man who attacked him.

A man who stabs Edward Burton after being bullied, and who later holds a grudge against those involved in Burton's recovery.

Cora's late husband and Francis's father. A civil servant who maintained a restrictive and unhappy marriage with Cora, prompting her to seek freedom in the countryside after his death.

A disabled beggar who gives Cora and Martha their first detailed account of the Essex Serpent myth, explaining how the creature was supposedly freed by a local earthquake.