Alex is a fifteen-year-old boy whose peaceful life in California is upended when his mother becomes severely ill. A skilled mountain climber and talented flutist, he struggles to manage his anger and fear regarding his family's situation. When his parents send him to accompany his grandmother on an Amazon expedition, he initially resists the unfamiliar environment. Over time, he begins to open his mind to the spiritual and natural wonders of the jungle.

Nadia is the intuitive, honey-colored teenage daughter of the expedition's Brazilian guide. She possesses a deep, near-mystical connection to the natural world, speaking several languages and communicating easily with indigenous tribes. Unlike the highly rational Alex, she is trusting and imaginative, guided by the teachings of a local shaman. She rarely travels without her tame monkey companion, Borobá.

Kate is Alex's eccentric, sixty-four-year-old paternal grandmother and a renowned travel writer for International Geographic magazine. She maintains a gruff, tough-love exterior, refusing to coddle her grandson while occasionally showing subtle signs of affection. She travels to the Amazon to document the existence of a mysterious, lethal creature known only as the Beast.

Mauro is a wealthy, powerful entrepreneur with international business dealings and homes in several countries. He originally encouraged the magazine to send a team to investigate the Beast. He views the undeveloped Amazonian land as wasted potential, seeking to exploit its natural resources despite the presence of indigenous tribes, and works actively to clear the area for his own gain.

Captain Ariosto is the commander of the local military forces in Santa Maria de la Lluvia, officially charged with protecting the nearby native tribes. He shares a hidden agenda with Mauro Carías, prioritizing personal wealth and control over the region's resources at the expense of the people he is meant to guard.

Ludovic is a fifty-year-old anthropologist who claims extensive expertise regarding native Amazonian tribes. He presents himself as a stereotypical jungle adventurer and frequently refers to himself in the third person. He holds a harsh view of the native peoples, considering them brutal and treacherous, and complains about the expedition's physical discomforts.

Dr. Torres is a beautiful young Venezuelan doctor traveling with the National Health Service. She joins the expedition with the stated goal of vaccinating native Amazonian tribes against deadly diseases brought by outsiders. She frequently defends the native populations against Ludovic Leblanc's harsh criticisms, viewing the tribes as peaceful and highly vulnerable to modern encroachment.

César is the hired Brazilian guide for the expedition and Nadia's father. He maintains a calm, knowledgeable presence throughout the journey, bringing a deep respect for the natural environment. He acts as a mediator between traditional native cultures and the arriving outsiders, helping the group navigate the physical dangers of the dense rainforest.

Walimai is an ancient, ethereal shaman who appears to have lived far longer than a normal human lifespan. He travels through the jungle accompanied by the spirit of his wife. He serves as a spiritual mentor to Nadia, providing her with a protective amulet and delivering cryptic warnings about the impending dangers facing the native tribes.

Mokarita is the ancient, kindly chief of the elusive People of the Mist. He wears a crown of yellow feathers and strictly upholds the traditional, communal lifestyle his tribe has maintained for thousands of years in their hidden valley.

Iyomi is a very old member of the People of the Mist and the wife of Chief Mokarita. She is a respected elder within the tribe who steps into a profound leadership position during a time of unprecedented crisis and change for her people, balancing ancient traditions with the sudden arrival of outsiders.

Karakawe is an indigenous man who has spent significant time among outsiders, adopting some of their customs. He works as a personal assistant to the anthropologist Ludovic Leblanc, carrying his supplies and tending to his needs. He maintains a brusque, solitary demeanor and frequently behaves suspiciously around the expedition's medical supplies.

Timothy is a laconic English photographer working for International Geographic. He travels to the Amazon to capture images of the legendary Beast and the surrounding landscape, bringing his assistant along to help document the harrowing journey for the magazine.

Padre Valdomero is an eccentric priest who originally came to the Amazon as a missionary but ended up living among the native tribes for years. He holds deep reverence for indigenous cultures and insists that outsiders pose a far greater threat to the natives than the natives do to outsiders.

John is Alex's father, a doctor who struggles to manage the household while his wife undergoes cancer treatment. He relies on logic and practicality, though his wife's severe illness clearly frightens him and forces him to send his son away.

Lisa is Alex's artist mother, who is battling a severe form of cancer. Her illness and impending travel to Texas for experimental treatment serve as the catalyst for Alex's journey to the Amazon, and her memory provides him with emotional strength.

Andrea is one of Alex's younger sisters. She is sent to stay with her maternal grandmother while her parents travel to Texas for medical treatments.

Nicole is Alex's youngest sister, who also goes to stay with her maternal grandmother when the family is forced to separate due to their mother's illness.

Morgana is a bedraggled girl slightly older than Alex. She presents herself as a vulnerable traveler in need of bus fare but quickly proves to be ruthless and opportunistic when she abandons him in the city.

Joel is an assistant photographer working with Timothy Bruce. His journey is cut short when he is severely injured by an anaconda while swimming in the river, forcing him to turn back early.

Matuwe is a local indigenous guide hired to assist the expedition as they travel up the Orinoco river. He shares information about native customs with the outsiders and helps navigate the local waterways.

Tahama is an honored hunter among the People of the Mist. He is deeply protective of his tribe and fiercely resistant to allowing outsiders near their hidden valley, trusting only the guidance of his elders.

Borobá is a small, tame monkey who behaves like an extension of Nadia herself. He travels everywhere with her, providing comfort and occasionally drawing her attention to hidden dangers in the jungle.