Stella is an adventurous woman who is willing to go through danger to help others. She's dedicated to her work, as shown when she creates a cure for the sleeping sickness. She is also shown to be rather tough, with threatening to punch Eduardo and getting mad at Miles in certain situations.
Stella is a doctor/scientist in botany, the branch of biology that involves the scientific study of plant life. She also has some degree of medical expertise, being able to successfully make a cure for an illness.
Stella has brown hair, an athletic build, then a slim figure, and typically wears a green tank top and blue pants. In "Parents Day", Stella's head is ovular, whereas it is more circular in her later appearances. Regardless, it is very clear that Arnold has inherited his "Football Head" from her.
In The Jungle Movie, which is her current appearance, Stella has grey streaks in her hair as a sign of aging. Other than that, she looks very much like she did 10 years ago. Her hair is also noticeably longer in the movie compared to her previous appearances (in old photos as well, not just her present look).
Stella is Miles' wife and Arnold's mother. She happened to be in the jungles of San Lorenzo (studying the local plant life to find cures for jungle diseases, as she said) when Miles literally fell into her life. The two soon became a couple and started exploring together, with the first place being a San Lorenzo temple that was dedicated to "the sacred spider." During a hike up to San Lorenzo's highest point, they both slipped down into a river and fell down a waterfall; only to be caught in a device set up for catching fish. They discovered that the net had been constructed by the mysterious Green-Eyed People; the original inhabitants of San Lorenzo.
To pay back the Green Eyes for saving them, Stella and Miles recovered their sacred relic, the Corazón, from the river pirate and treasure hunter, Lasombra. As a reward, the couple was accepted as members of the Green Eye family. Shortly afterward, Stella married Miles in a wedding ceremony at an ancient Mayan temple. Their honeymoon was in Rio de Janeiro in Brazil.
The Green Eyes were then infected with the sleeping sickness and it was up to Stella and Miles to create an antidote for the illness. They were able to do this and saved the Green Eyes just in time.
Not long afterward, Stella became pregnant, and months later gave birth to Arnold, who she named after her late father. Due to the dangers surrounding the jungles, Stella and Miles decided to go back to Hillwood. However, over a year later, Eduardo returned to ask them for one more mission back in San Lorenzo. Stella and Miles never returned from their mission, and their fates were unknown over the next 9 to 10 years.
In The Jungle Movie, Arnold, Helga, Gerald, and Eduardo find the Green Eyes' city, where they discover that Stella and Miles had caught the sleeping sickness before they could distribute the medicine and return home. Although alive, Stella, Miles, and other adults in the city can only be awakened with help from the now-lost Corazón. However, when Helga decides to use her locket as a replacement, it works, and Arnold and all of the children reunite with their revived parents.
A few months later, Stella and Miles are revealed to truly be alive and back home with Arnold after he wakes up on the morning of his first day of sixth grade. Stella suggests the three of them go on an adventure to the city's botanic garden, but Arnold reminds her and Miles that he has to go to school. Shortly after Arnold leaves with his friends, Stella and Miles follow them, asking Arnold if they can walk him to school, to which Arnold agrees. When they arrive, Arnold tells his parents that he'll be done at 3:30. Stella assures him that she and Miles will be waiting on the steps before Arnold smiles at them and the school doors close behind him.