She has golden hair and blue eyes. Combing her hair always makes her cross, and she's never at her best in the morning.
She has large round handwriting.
As a child, Gwendolen lived with her parents and brother upstairs at No. 26 Coven Street, Wolvercote. After their parents were killed in the Saucy Nancy Disaster, they were taken in by Mrs Sharp, who lived downstairs at No. 26, and the town of Wolvercote undertook to pay for their education and upbringing.
Mrs Sharp declared Gwendolen to have a great talent for magic, and arranged lessons for her with Mr Henry Nostrum. The town declined to pay for these lessons, on the grounds that magic was not part of an ordinary education (Mrs Sharp declared that it was really prejudice against the different), and the lessons ended up being paid for with three letters bearing the signature of Chrestomanci that were among the late Francis Chant's belongings. Mrs Sharp put her down as having music lessons, and got an allowance from the town council for that, but kept the money herself; when Cat took her up on the subject, she arranged for him to have music lessons instead.
Gwendolen rather neglects her more mundane studies, which she finds boring, with the result that her grasp of such things as history, mathematics, and spelling are rather poor.
As Gwendolen's fame spreads through the neighbourhood, she is made a pet of by the other magic-users in the street, who give her presents. The Fortune-Teller a couple of doors down tells her fortune, which includes a change for the worse and a dominant Dark Stranger, but she will rule the world if she goes the right way about it.
Not long after that - about a year after their parents' deaths - Cat and Gwendolen are taken to live at Chrestomanci Castle with Chrestomanci and his family (who are cousins of theirs, although it is some time before anybody thinks to explain this). It soon becomes clear that Chrestomanci is the dominant Dark Stranger of Gwendolen's fortune; she chafes under his restrictions, especially his decision to shelf her further magical education until she's had a chance to unlearn some of the bad ways she picked up from Mr Nostrum.
Over the course of the story, we learn something about Gwendolen's alternates in other worlds, who include Caroline, Janet, Jennifer, and Romillia. Some of what we learn about them is Charmed Life (spoilers).