John is the 12- or 13-year-old captain of the Swallow and the eldest Walker sibling. He is brave, highly competent in sailing and camping, and plans to join the Navy when he grows up, mirroring his father. Driven by a strong moral code, he takes his responsibilities to his crew seriously and strives to act honorably in all his dealings.

Susan is the 11- or 12-year-old mate of the Swallow. She serves as a maternal figure for her younger siblings, taking great pride in cooking, cleaning, and organizing the camp. Alongside these practical domestic skills, she is an expert sailor who adeptly hoists sails and steers the boat, functioning as John's reliable second-in-command.

Titty is the 9- or 10-year-old "Able-seaman" of the Swallow. Highly imaginative and enchanted by tales of pirates and buried treasure, she draws heavy inspiration from classic novels like Robinson Crusoe and Treasure Island. Her adventurous spirit frequently blurs the line between reality and fantasy, driving much of the group's imaginative play.

Roger is the seven-year-old "Ship's Boy" of the Swallow. Eager to participate in his older siblings' games, he is generally brave and helpful, though he occasionally experiences moments of fear when imagining sharks or octopuses in the lake. He is highly motivated to prove his worth to the older children.

Nancy is the impulsive and fierce captain of the Amazon. Having rejected her given name because true Amazons are "ruthless," she fully embraces a piratical persona inspired by Treasure Island. She is a natural leader who loves dangerous play and plotting mock attacks against her grumpy uncle.

Peggy is the mate of the Amazon and Nancy's younger, more innocent sister. Less brave than Nancy but eager to follow her lead, she has a habit of blurting out secrets and earning affectionate insults from her sister. Despite her minor fears, she happily participates in the Amazon crew's piratical activities.

Jim is the uncle of the Blackett girls and the owner of the local houseboat. Dubbed "Captain Flint" by the children, he is spending his summer trying to write a memoir and has turned reclusive and irritable. His desire to be left alone leads to conflict with the young sailors, who incorporate him into their games as a pirate enemy.

Mother is an enabling and supportive parent who easily falls into the children's imaginative play, using nonsensical words and treating them as real explorers. She provides vital supplies, checks on their safety, and is a highly capable sailor herself, having grown up near Sydney Harbor.

Commander Walker is a British Navy officer currently stationed in Malta and heading to Hong Kong. Though physically absent, his naval code of honor heavily influences his children's conduct on the island.

Vicky is the two-year-old baby of the Walker family. Because she is so young, she remains at Holly Howe farm while her older siblings camp on the island.

Mr. Jackson is a local farmer at Holly Howe who assists Mother in transporting gear and checking on the young campers across the water.

Mrs. Dixon is a farmer's wife who interacts with the children when they fetch their milk. She is a practical, prosaic adult who is quick to pass along Jim Turner's accusations and dismisses the children's imaginative dreams.

Billy is an older charcoal burner working in the nearby hills. He keeps a pet adder snake for luck and treats the children kindly when they visit his camp.

Young Billy is a charcoal burner who helps his father in the hills. He gives Susan practical tips on maintaining a campfire overnight and asks the children to pass an important warning about local gossip to Jim Turner.

Sammy is a local policeman whose mother was once a nurse to the Blackett family. He attempts to investigate a crime on the lake but is easily intimidated by Nancy Blackett.