Kanai is a 42-year-old, upper-class translator from New Delhi who speaks six languages. He travels to the isolated Sundarbans to visit his aunt and review his late uncle's newly discovered journal. Confident and highly observant, he moves through the world with a sense of casual self-importance, assuming his education and urban background make him superior to rural locals. His journey forces him to confront his own class privilege and the harsh realities of the tide country.

Piya is an independent, American-born cetologist of Indian descent traveling to the remote Sundarbans to study a rare species of river dolphin. Raised in Seattle by immigrant parents, she struggles with feelings of isolation, fitting in neither with her white peers nor traditional Indian culture. She possesses a strong sense of justice, prioritizing the preservation of wildlife and the fair treatment of locals over her own safety.

Nilima is Kanai's pragmatic and tenacious aunt. She runs the local hospital in Lusibari and founded a women's Trust to help widows gain financial independence. She commands immense respect within the local community due to her decades of dedicated social work. She calls Kanai to the islands after discovering a sealed packet of writings left by her late husband.

Fokir is an illiterate but highly skilled local fisherman with an intuitive connection to the rivers and wildlife of the Sundarbans. He rescues Piya from drowning and becomes her essential guide in tracking the elusive river dolphins. Though he lacks formal education, his deep understanding of the natural world and quiet compassion make him a formidable presence on the water.

Kusum is Fokir's mother and Kanai's childhood playmate. Rescued from extreme poverty and human trafficking by the Trust, she grows into a fiercely principled and independent young woman. She pushes back against Kanai's childhood arrogance and eventually becomes a central figure in Nirmal's secret journal chronicling the events on Morichjhapi.

Moyna is Fokir's ambitious and highly practical wife. Working as a trainee nurse at Nilima's hospital, she values education, modernization, and stability. She is frequently frustrated by her husband's traditional, dangerous lifestyle on the river and is determined to secure a better, educated future for their young son.

Nirmal is Kanai's late uncle and Nilima's husband. Formerly the headmaster of the Lusibari school, he was an intellectual from Kolkata who relocated to the Sundarbans due to his Marxist political leanings. He leaves behind a hidden journal detailing his profound connection to the local people and his involvement in early political struggles.

Tutul is the young son of Fokir and Moyna. He frequently accompanies his father on fishing trips, displaying a natural comfort on the water. His mother desperately wishes to keep him off the boats so he can attend school and escape the dangerous life of a fisherman.

Horen Naskor is an experienced local boatman in the Sundarbans. He is a longtime friend to Nirmal and a fierce protector of Kusum, frequently ferrying characters across the treacherous rivers. He possesses practical survival skills and a deep knowledge of the tide country's hidden dangers.

Mej-da is the unscrupulous owner of a launch vessel hired by a forest guard to transport Piya. He acts crudely and attempts to exploit Piya's vulnerability as a lone foreign woman, leading her to eventually abandon his vessel.

Sir Daniel Hamilton is a visionary Scottish businessman from the past who bought vast tracts of land in the Sundarbans. He attempted to build a utopian, classless society by inviting land-starved Indians to settle the dangerous tide country.

Lucy McKay Hamilton was the niece of Sir Daniel Hamilton. The village of Lusibari is named in her honor, though she died in a shipwreck before ever seeing the settlement.

Rajen is a disabled man originally from the tide country who lives in a mining town. He provides shelter for Kusum, helps her locate her mother, and briefly marries her before dying in a work accident.

Rath is a local government official in Cambodia whom Piya briefly dates during an internship. After earning her trust, he marries another woman and spreads Piya's intimate secrets throughout his village, leaving her deeply guarded about future relationships.

Dilip Choudhury is a local man operating out of Calcutta. He functions as a pimp and human trafficker, attempting to exploit a young Kusum by luring her to the city under false pretenses.