Elizabeth "Eliza" Thornberry is an extremely cute and unbearably beautiful 13-year-old tomboy whom, unknown to most, except her older sister, Debbie (as of the movie), gained the ability to speak with animals after an encounter with an African shaman, two years prior to the start of the series at age ten. Despite her cuteness and beauty, Eliza is portrayed as a geek by her extensive zoological knowledge and her glasses, braces and by occasionally standing pigeon-toed. She has freckles and shoulder-length red-orange hair that is usually worn in braided pigtails with two red elastic bands and occasionally a ponytail. She almost always wears a yellow dress, though later in the series she sports other clothes.
Eliza is an 13-year-old girl, who is always ready for adventure. Her natural curiosity means she often leaps before she looks often with amusing or disastrous results. She possesses the power to speak clearly to animals and understand their reactions. She loves animals and exploring the wild jungles, deserts, and forests that she travels to along with her family all around the world as they film their nature television show: Nigel Thornberry's Animal World and Sir Nigel Thornberry's Animal World post 'Sir Nigel Part 2'. What her family doesn't know is that she was given the ability to talk to every animal she meets by a shaman in Africa. Eliza's "Gift of Gab" is a secret. If anyone ever found out she speaks to animals, she would lose her powers. It isn't easy for Eliza to keep such a big thing to herself, but luckily she has her best pal Darwin to confide in. Eliza has saved numerous animals from poachers, trappers, and other perilous situations, and never fails to make new friends, no matter what species they may be from. She gains her love of adventure from her parents (her father mostly), which often leads to problems. Eliza is the only member of her immediate family to have their appendix removed (Operation Valentine). Because of her gift, Eliza goes to extreme efforts to talk to all kinds of animals and get to know them, even if they're predators. This obsession has often led to mishaps or Eliza foolishly putting herself (and sometimes others) in danger. Eliza's reckless behavior stems from her naive perspective that, since she can talk to animals, she knows them better than actual experts and plenty of times she has been proven wrong; she never seems to learn her lesson. In an eleven year fast forward edition of Nick Magazine, Eliza is shown to be a documentary host (At 23), like her parents, her show is the second most watched behind her fathers.
Eliza almost always wears a short-sleeved yellow dress with a red collar, buttons, cuffs and pockets, white panties, a pair of brown boot-like shoes, and a pair of glasses without which she can't see. She has orange hair in pigtails, sometimes she wears her hair in a ponytail and braces. Upon occasion, Eliza will let her hair down from the pigtails/ponytail. and when she does her hair is longer than her moms but shorter than her sisters. In episodes in which Eliza wears bathing suits, her outfit consists of a purple bathing suit with brown shorts and purple flip-flops. In the cold climates, she wears a thick parka which is purple or blue in color. In episodes Eliza wears her scuba gear, her wetsuit is yellow much like her everyday outfit a mask on her face, flippers on her feet, a red air tank on her back to carry air to breathe underwater, and a regulator in her mouth. As a 22-year-old woman, Eliza keeps her red hair in a ponytail, and switch from her large glasses to smaller and oval shaped ones (much like her mothers). She wears a safari outfit, with long green socks and a red bandana tied around her neck with a watch on her left wrist.
Eliza, being the main protagonist of the series, has appeared in every episode of The Wild Thornberrys. Eliza was also the main protagonist of The Wild Thornberrys Movie. However, despite being the main character of the series, she was a secondary character in Rugrats Go Wild! due to Tommy Pickles idolizing Nigel.
Marianne: Eliza loves her mother and would do anything for her. Eliza's and Marianne's day is April Fool's Day on which Eliza is Marianne's partner in pranks. Marianne was being too strict to Eliza.
Nigel: Eliza loves her father who was happy and sweet to his favorite daughter. Nigel generally calls Eliza "poppet" or "poodles". Nigel doesn't like it when his wife being too harsh on Eliza and he also doesn't like rules if he's with Eliza.
Debbie: Eliza loves her sister but has a hard time showing it. Eliza and Debbie often get into arguments. In Operation Valentine after Eliza and Debbie ended an argument about Debbie's cookies and Eliza's Secret Admirer, Nigel comments "It would be far easier to raise crocodiles", referring to how hard it was to raise two girls in the wild.
Donnie: Eliza loves Donnie and accepts him as her brother. Donnie often accompanies Eliza on her adventures. Donnie makes Animal sounds that even Eliza can't understand. In Operation Valentine, Donnie finds Nigel to help Eliza get back to camp.
Darwin: Darwin is Eliza's best friend. She once called him a "Cry Baby in a Tank Top". Darwin carries Eliza down a mountain in Operation Valentine, due to Eliza's Appendicitis.
- Eliza has seemed to inherit her bad vision from her mother, but it is unknown how bad Marianne's vision is.
- Eliza has had several near-death encounters both with animals, other humans and nature itself.
- Eliza bears some resemblance to Pippi Longstocking; the pigtails, an ape companion, a 'power' that sets them apart from others.