Georgina is a fiercely independent eleven-year-old girl who actively rejects traditional femininity, wearing boyish clothing like jeans and jerseys while maintaining a short haircut. As the informal owner of Kirrin Island and its ruined castle, she is initially territorial and solitary, preferring the company of her secret dog over other children. Her superior physical capabilities, from swimming to steering rowboats through rocky waters, make her an indispensable guide for her cousins. Despite her initial hostility, she operates with a strict code of honor and deep loyalty to her friends.

As the eldest of the visiting cousins, Julian acts as the natural leader and primary strategist of the group. He proposes the initial bargain of sharing treats in exchange for seeing Kirrin Island, successfully turning his hostile cousin into a collaborative friend. He takes his responsibility seriously, ensuring the group brings necessary supplies like ropes, spades, and torches before setting out on expeditions. His calm, level-headed demeanor grounds the children during dangerous situations underground.

Dick is Julian's practical and action-oriented younger brother. He brings physical courage and sharp perception to the group's adventures, often noticing crucial psychological details that others miss. He is the first to recognize when a note signed with an incorrect name indicates a trap set by hostile adults. He excels in tactical, hands-on situations, readily climbing down deep, abandoned well shafts and confronting perilous physical challenges to ensure his friends' safety.

Anne is the youngest sibling, characterized by her preference for dolls and dresses, which initially alienates her unconventional cousin. She is frequently frightened by dark dungeons and stormy weather, preferring domestic tasks like preparing the group's picnic lunches. Despite her timidity, she demonstrates remarkable emotional maturity by initiating reconciliation after arguments. Her accidental discoveries and instinctual questions frequently provide critical breakthroughs for the group when formal planning fails.

Timothy is a large brown mongrel dog secretly kept by a local fisher-boy after being permanently banned from Kirrin Cottage. He possesses keen sensory awareness, instinctively tracking rabbits to uncover hidden passages and alerting his companions to incoming threats. As a vital member of the adventuring group, his physical interventions often bridge the gap between the children's rational plans and the unpredictable natural environment.

The Villains are a group of greedy men, led by a man named Jake, who attempt to exploit local historical artifacts for personal wealth. They initially disguise their intentions through legal purchases, buying antique items and attempting to acquire private islands for commercial development. When legal avenues prove insufficient, they readily resort to intimidation, kidnapping, and physical force to secure the hidden gold.

Uncle Quentin is a temperamental and financially struggling scientist who commands a tense household at Kirrin Cottage. He is deeply absorbed in his academic work, viewing the visiting children mostly as a noisy nuisance to be managed or restricted. His short-sightedness and focus on immediate financial relief make him dismissive of youthful insight, leading him to carelessly sell important historical artifacts without examining them.

Aunt Fanny is a kind, nurturing mother figure who provides domestic stability in a household otherwise defined by scientific obsession and financial strain. She takes on her relatives as paying boarders to help alleviate the family's monetary issues and tries her best to mediate domestic tensions. Though sympathetic and welcoming, she remains entirely unaware of the dangerous activities occurring right off the coast.

Alf is a local fisher-boy living in the coastal community around Kirrin Bay. He acts as a secret caretaker for a dog that has been banned from the local cottage, faithfully hiding and feeding the animal in exchange for all of the owner's pocket money. He possesses good knowledge of the local weather patterns and coastal dangers, warning the children of incoming storms.