The series focuses around a nomadic family of documentary filmmakers known as the Thornberrys, famous for their televised wildlife studies. It primarily centers on the family's younger daughter Eliza, and her secret gift of being able to communicate with animals, which was bestowed upon her after having rescued a shaman masquerading as a trapped wild animal. The gift enabled her to talk to the Thornberrys' pet chimpanzee Darwin. Together, the pair frequently venture through the wilderness, befriending many species of wild animals along the way or realizing moral truths and lessons through either their experiences or a particular animal species's lifestyle, or simply assisting the creatures with which they become acquainted in their difficulties
Eliza is the younger daughter of the Thornberry family. She is intelligent, true-hearted, fearless, and good-natured. Having been given the ability to communicate with wildlife, she often employs this skill when embarking on explorations through the wilderness, traveling around the world with her parents, or encountering different wild creatures. She is forced to keep her gift secret from her family, having been warned that she will lose her powers if she tells anyone about them (although Debbie learns the secret during the events of the movie). Aided by her intelligent pet chimp, Darwin, she often sets out to assist or meet countless differing animals. Eliza is depicted in a stereotypically-nerdy fashion, with red hair plaited in braids, eyeglasses, braces, and freckles. She is a scuba diver. Eliza is voiced by Lacey Chabert.
Darwin is a chimpanzee who travels with the family and is a loyal companion to Eliza. He usually wears a tank top with horizontal blue and white stripes (used to be Debbie's lucky tank top) and blue shorts. He speaks with a distinct upper-class British accent. He tries to be the voice of reason to Eliza, however she rarely takes his advice. Cautious by nature, he is almost always hesitant to follow her plans; in response, she accuses him of cowardice, once calling him a "crybaby in a tank-top" for his hesitancy. He was found by Eliza, and was the first animal she ever talked to. In the Wild Thornberrys Movie: The Origin of Donnie, Darwin was seen in a flashback, stealing the sleeveless tank top that was once owned by Debbie. His name is most likely a reference to Charles Darwin, though within the series Eliza gives him the name Darwin after Lenny Darwin, her cousin who looks like a monkey (is never shown in the series). Darwin is voiced by Tom Kane.
Donnie is a feral boy who (as revealed in the 2001 4-part TV movie, "The Wild Thornberrys: The Origin of Donnie") was raised by orangutans in Borneo. His real parents, Michael and Lisa, were naturalists who loved and protected animals with their hearts. One night, it is revealed, two poachers attacked a pair of orangutans, intending to harm or kill the mother and sell the baby on the black market. Donnie's parents managed to save them but are killed for interfering, much to the horror of the orangutan mother and the local villagers who came to know about their deaths. Out of gratitude and love, the mother orangutan took Donnie and raised him as a son. One day, however, she comes across the Thornberrys and decides to give Donnie to them so he can have a human family. She watches tearfully along with her son as he is welcomed into their family as their own. Before the Thornberrys officially adopt him, it is revealed that Nigel and Marianne knew his parents as they had met them while Donnie's parents were on their honeymoon. He usually wears a pair of leopard-spotted shorts (similar to Tarzan's leopardskin loincloth) and he loves to eat all types of bugs. He babbles wildly and makes animal-sounds (but nothing Eliza can understand), occasionally spouting bits of English or exhibiting intermittent signs of sophistication. Donnie is 4, turning 5 in "The Origin of Donnie", and it is presumed he does not know what happened to his parents. It is revealed that he was able to pick up sign language from watching his parents teaching it to orangutans. We only see him sign one word (family), and it is unclear how much he knows. Nevertheless, this proves Donnie's intelligence - something which the audience sees further evidence of in a drawing he does of his parents alongside the orangutan family that raised him. Donnie is voiced by Flea of the Red Hot Chili Peppers.
Debbie is the eldest child of the Thornberrys, depicted as a typical teenage girl. Captivated with fashion and music, Debbie is portrayed as considerably more self-absorbed than her sister Eliza and possesses the tendency to whine and complain, particularly over her family's nomadic way of life and her desire to lead an average life. She really gets on the family's nerves and annoys them a lot. She shows no interest in nature whatsoever and is far more concerned with the stereotypical interests of an adolescent girl, frequently moody or disgruntled in annoyance with the Thornberrys' endless travels around the continents. She also shows disdain towards Eliza, who is mutually infuriated with Debbie's own self-absorption, but nonetheless cares for her sister deep down, most notably in the movie in which she becomes the only family member to know about Eliza's powers but is later warned that she will be turned into a baboon if she ever tells anyone. Debbie is voiced by Danielle Harris
Nigel is the child of Radcliffe and Cordelia Thornberry (Lynn Redgrave), an aristocratic British couple. Born in Britain himself, he travels around the world with his own family making wildlife documentaries. He may be inspired by Sir David Attenborough and speaks with a nasally RP English accent. He is eccentric, enthusiastic and cheerful, even in the face of danger. He was offered a professorship at Oxford University but, much to his mother's disapproval, Nigel turned it down. He is an expert zoologist, as well as naturalist, traveler and survivalist and is quite caring and logical though Nigel often otherwise appears absent minded or naive. In the series, Nigel is a supporting character, but in the film Rugrats Go Wild, Nigel plays a much larger role as he is idolized by Tommy Pickles (who usually refers to him as "Mr. Strawberry"). Nigel is voiced by Tim Curry.
Marianne is Nigel's wife, daughter of Frank (Ed Asner) and Sophie Hunter (Betty White). She operates the film camera and edits their nature show. Marianne is a strong woman, often the voice of reason, and sometimes loses patience with Nigel. She is usually the one to take on disciplinary duties, trying to keep the girls out of trouble and finish their homework. In Season 1, Episode 12, she mentions that she attended the University of California, Berkeley. Marianne is also a scuba diver like Eliza. She wears a wetsuit, flippers, mask, and scuba tank in Season 3, Episode 17. Marianne is voiced by Jodi Carlisle