She is courageous and seems very calm and collected. When Amy and Dan tell her about the "treasure hunt" in Maze of Bones, she reacts as if it's normal. Nellie loves music and rarely takes her iPod's headphones out of her ears. She also loves cranking it up to brain damage level. She can be mischievous, as it is shown when she shakes up soda and opens it in Natalie's face. Nellie loves cooking, a trait she shares with her dad and her boyfriend, Sammy Mourad.
Nellie was hired as an au pair for Amy and Dan Cahill by their grandmother, Grace, in late 2007. She was the perfect candidate, her family having guarded a Cahill treasure (the snake earring as shown in Storm Warning) and also having the necessary skills. Aunt Beatrice is threatened by Grace that she will not get part of her will if she fires Nellie. She only keeps this promise as long as Grace is alive, as by the time she passed away in the Maze of Bones, on the way to Grace's funeral, Aunt Beatrice says, "Remind me to fire your au pair as soon as we return to Boston." Nellie accompanies Amy and Dan throughout the Clue Hunt. She is kidnapped by the Vespers and shot in the shoulder while in captivity, later having the bullet removed by Natalie Kabra. During Unstoppable, she takes the name of Nadine Gormley, a brilliant young chemist, in order to spy on Trilon Laboratories where Sammy Mourad is being kept. During the events of Doublecross, Nellie and her now boyfriend Sammy Mourad travel to different cities and branch strongholds to try and find out more about The Outcast and stop his attacks. By the time of Superspecial, she owns a restaurant, and helps Dan and Amy with missions.
Nellie has a father, Pedro Gomez, a mother (unknown) and a sister (unknown). She is also the legal guardian of Amy and Dan Cahill, along with Fiske Cahill. She was thought to be an employee of Beatrice but in reality, she was hired by Grace. She is a distant relative of Lester and Miss Alice.
Nellie has been described as having a silver snake earring in her nose, and multicolored hair. She has light-medium skin and dark brown eyes. In the Maze of Bones, she is recorded to have brown and blond hair, but these changes throughout the series as she is fond of dying her hair. Her natural hair color seems to be brown, as it is that color when she washes her hair dye out in Unstoppable.
Nellie is the au pair, later official guardian (along with Fiske Cahill) for Amy Cahill and Dan Cahill. She agrees to help them on their quest for the 39 Clues, traveling all around the world. In the Maze of Bones, Dan comments that he likes Nellie because she "makes amazing waffles and cranks her iPod up to brain damage level." Nellie takes care of the cat, Saladin, an Egyptian Mau formerly belonging to Grace Cahill. Nellie, because she grew up trilingual, is fluent in many languages. She uses this skill in the Clue Hunt to help Amy and Dan decipher the meanings of many Clues all over the globe. She grows closer to the two children she au pairs during the journey, becoming worried about frantic if they are gone off hunting Clues for more than a few hours. She is not that amazing to everyone she meets, however. The Kabras are disgusted by Nellie, their disrespect for her increasing after she sprays her Coke in their faces after Natalie accidentally spills her whole vial of poison into it during turbulence. She is close to Fiske Cahill, because of their shared guardianship of Dan and Amy, and they treat each other like family. She is also Sammy Mourad's girlfriend.
Nellie's main card is Card 244.
In the deluxe diamond edition, you can see her with alternate hair colors.
- Nellie Gomez grew up multilingual, knowing French, Spanish, and English from an early age due to her mother being French, her father is from Mexico, and she knew English because she lives in the US. She also has a "working knowledge" of German and Italian.
- She was the first Madrigal to receive Madrigal status while she was not born in the bloodline.
- Two different actors portray her on her cards.