Ralph is a fifteen-year-old boy from an English fishing village who possesses an insatiable curiosity and a deeply contemplative nature. Raised by a sea captain father and a pious mother, his worldview relies heavily on his Christian faith. As the narrator, he is observant and earnest. He views the South Seas adventure as both an educational and spiritual journey, frequently reflecting on the natural wonders of the coral reef.

Jack is an eighteen-year-old boy who acts as the unquestioned leader of the shipwrecked trio. Tall, strong, and highly intelligent, he uses his extensive reading of travel and adventure books to survive the dangers of the South Seas. He is pragmatic and resourceful, transforming raw materials like hoop-iron and tree roots into tools and watercraft. He protects his younger companions with a calm but unyielding demeanor.

Peterkin is a fourteen-year-old boy who provides constant energy and comic relief for his friends. Small, mischievous, and buoyant, his optimism rarely falters, even when stranded on a remote island. While he initially lacks Jack's practical knowledge and Ralph's solemnity, he possesses a core of unwavering loyalty and a surprising capacity for bravery during violent encounters.

The unnamed captain of the pirate schooner is a cruel, self-serving leader who embodies the destructive forces of greed. He maintains a meticulously clean ship and presents himself deceptively as a legitimate sandalwood trader. In truth, he enforces his authority through fear and unprovoked brutality, regularly firing his cannons upon indigenous populations over minor disputes.

Bloody Bill is a massive, hardened crewman aboard a pirate schooner operating in the South Pacific. Worn down by years of complicity in brutal activities, he harbors a lingering conscience and a capacity for compassion. He acts as a cynical but practical guide to the dangerous reality of the islands, warning newcomers about local customs and the violent nature of European expansion.

Tararo is an imposing indigenous chief who commands significant authority among the islands. He is a formidable warrior deeply entrenched in the customs of his people. Operating by a strict code of conflict and sacrifice, his practices repeatedly put him at odds with the British boys' moral framework when they cross paths.

Avatea is a young Samoan woman held captive by Chief Tararo. Distinguished by her lighter skin, gentle demeanor, and an inclination toward Christianity, she is treated as property by the island's leaders. Her perilous situation serves as a moral catalyst for the boys' interventions in the region.

The Native Teacher is an indigenous missionary convert who lives on the southern side of the island of Mango. Pious, intelligent, and articulate, he dresses in European clothes and speaks fluent English. He acts as a bridge between the white protagonists and the local islanders, providing necessary exposition about local politics.

Romata is a powerful chief on the island of Emo who trades sandalwood with visiting European ships. He rules his people with unchecked violence and rigid adherence to local taboos. His volatile temperament and willingness to brutalize his own subjects complicate diplomatic and commercial relations.

Ralph's mother is a devout Christian woman living in an English fishing village. Before her son departs for the South Seas, she provides him with a Bible and secures his promise to read it daily. Her parting instructions establish the moral compass that guides Ralph through his trials.

Ralph's father is a retired sea captain who settled down after a life on the Atlantic Ocean. Understanding his son's roving spirit, he arranges for Ralph's early apprenticeship on a coasting vessel and later facilitates his major voyage to the Pacific.