Barney is the youngest of the Drew siblings. He is an eager, curious boy who brings a deep passion for Arthurian legend to the family's summer vacation. He frequently models his behavior on the heroic knights he reads about, using his historical knowledge to process unfamiliar and frightening situations.

Jane is the middle child of the Drew siblings. She possesses strong emotional intuition and quickly detects hidden motives in the ostensibly friendly adults around the village. Her natural caution balances her brothers' recklessness, leading her to assess situations carefully before taking action.

Simon is the oldest of the Drew siblings. He assumes command during the children's games and investigations, though his attempts to project authority sometimes mask his private insecurities. He feels directly responsible for his younger sister and brother and tries to ensure their physical safety during outdoor excursions.

Professor Lyon is a respected scholar and close friend of the Drew family. He possesses wild white hair and deep-set eyes, and he frequently vanishes on obscure archaeological trips. Rather than restricting the children's explorations, he actively encourages their interest in local history and geography.

Mr. Hastings introduces himself as the local vicar. He projects a grim, intimidating physical presence and shows an intense curiosity about the contents of Captain Toms's home. He employs a polite conversation style to quietly extract information from the children.

Norman is a man spending the summer on a luxury yacht moored in the Trewissick harbor. He maintains pleasant manners when interacting with the Drew parents but shows persistent curiosity regarding the local geography and the old artifacts in the Grey House.

Polly shares a luxury yacht with her brother in the local harbor. She attempts to ingratiate herself with the Drew children through cheerful conversation, aiming to learn exactly what they have discovered during their time in the village.

Bill is a local village boy who runs errands and transports supplies to the yachts in the harbor. He behaves aggressively toward the visiting children and shadows them as they walk around the surrounding hills.

Mrs. Palk is the local cook hired to maintain the Grey House during the holiday. She prepares meals and assumes basic caretaking duties when the parents leave the house.

Mr. Penhallow is an older Cornish fisherman working in the local harbor. He possesses extensive knowledge regarding the unusual local tides and currents, and he willingly shares this information with the visiting children.

Mr. Drew brings his family to Cornwall for a long holiday. He focuses on enjoying his vacation, quickly accepting invitations from the yachting crowd and leaving the children to amuse themselves around the village.

Mrs. Drew travels to Trewissick with her family. She spends her holiday painting near the harbor and socializing with local friends, trusting the children to stay safe while she occupies herself with her art.

Rufus is a dog belonging to Captain Toms, the absent owner of the Grey House. He accompanies the children on their outdoor excursions and reacts strongly to unfamiliar situations.

Miss Hatherton resides in Trewissick and maintains a friendship with Mrs. Drew. She invites the adult Drews to her home for the evening, keeping the parents occupied during dinner.