Jack is described as being in her late twenties, with tangled red hair and a round, boyish face, and is conveyed as being caring towards Alex, even if she isn't his legal guardian. The true nature of their relationship has been questioned and called more than extremely close. Though what is known is that she has been with (likely looked after) Alex since he was seven.
Eight years before the start of Stormbreaker, she came to Britain to study for law school and was a law student trying to be a lawyer. When there, she was trying to earn some money by shopkeeping, and eventually met the Riders. She struggled to receive a visa (mentioned by the MI6 director Alan Blunt) before Ian Rider employed her to look after Alex. After Ian, died she became Alex's legal guardian.
Role in series
Jack was Alex's legal guardian after Ian Rider died. She was in a way responsible for Alex to be a spy, because Blunt had persuaded Alex to take up spying by threatening to have Jack expelled from the country.
Alicia Silverstone portrayed her in the film version of Stormbreaker. While in the novels she is a red-head, in the movie and the graphic novels Jack is blonde.
Differences in the TV series
In the TV series, Jack Starbright is neither redhead nor blonde, as in the books and the film respectively, but rather a completely different race than all previous portrayals. She continues to fill the role of Alex's caretaker and great friend, with her worrisome calls to school or MI6 repeatedly resulting in benefit to MI6's efforts throughout the season. Before Alex begins cooperating with MI6, she is nearly deported, but is left alone when Alex relents and agrees to assist MI6.
Razim was intent on killing Jack, but Giovanni and Eduardo Grimaldi stopped Razim from doing so and decided that she was perfect for their own operation, Steel Claw. So they came to a compromise. Jack was tricked out of her prison cell in Razim's fortress and was led into a car. Then she drove out into the desert, looking for help to save Alex. Unbeknownst to her, an explosive was strapped on the bottom of the car. At the last second, guards forced her out of the car, and they let her watch as the car exploded. Razim then edited the tape to make it seem like that Jack was killed in the explosion, and used it to torture Alex. Alex first figured out he was tricked because a bird disappeared, which means that more time passed than Razim wanted Alex to think.
Alex found out that Jack was still alive in Never Say Die; the Grimaldi brothers had kidnapped her from the bomb-rigged car before it exploded, and had forced her to work as a nurse in their Operation Steel Claw. However, she managed to send a brief message to Alex, which led him from America to Siwa, thence to Saint-Tropez, thence to London, thence to Wales. He reunited with her, and together, they saved the fifty-two schoolchildren from the twins' plot, and moved back into their old house, with Jack resuming her law studies.
When Jack started working for Ian Rider, she was a college student. She took good care of Alex throughout the series.
Nobody ever knew what the name "Jack" stood for. Alex asked her a few times but she just changed the subject and didn't mention anything more.