Gwendolen is the first child and only daughter of Francis and Caroline Chant. The marriage was greatly frowned upon by their relatives because they are cousins. As her parents are both magical, they had a greater chance of producing children with witchcraft, and Gwendolen's manifests at an early age. Eric, also known as Cat, was born approximately two years after Gwendolen. Although she was very motherly to Cat in the past, she became nastier and bossier toward him over time. She is also related to Christopher Chant, also known as the Chrestomanci.
Gwendolen says in Charmed Life that she has been using Eric's magic ever since he was a baby. At the age of approximately five years, she placed Eric's lives in a matchbook for safekeeping. She used one of the eight remaining lives, due to the fact Eric had already lost a life at birth. She is also the person who gave him the nickname Cat, though it is unclear whether anyone but Gwendolen knew her reasoning for this.
Gwendolen's abilities do not go unnoticed by her parents, but due to Francis insulting the Chrestomanci of the time, Christopher Chant, her powers are unchecked and she continues to use Eric's enchanter magic for her own purposes.
When Gwendolen is about eleven, and Eric about nine, their parents die in a boating accident, called in the media the Saucy Nancy disaster. Eric and Gwendolen survive; Eric believes that this is because he is clinging to Gwendolen, who cannot drown because she is a witch. As one of his specters she called forth to the dinner is a drowned child, it is likely that Gwendolen used one of his lives to keep her from dying permanently.
After the boating accident, Eric and Gwendolen move in with Mrs. Sharp downstairs and Gwendolen begins magic lessons with Mr. Nostrum, a local necromancer. Eric gets violin lessons instead of magic, but eventually Gwendolen uses Eric's fourth life to turn his violin into a cat, simply out of irritation at his poor playing.
A Fortune-Teller reads Gwendolen her fortune and predicts she'll rule the world someday if she goes about it the right way. Miss Larkins, the neighborhood favorite before Gwendolen, tries to predict Eric's fortune out of jealousy. She goes into a trance, and a man's voice tells Eric how glad he is to have found him, but that he must be more careful: four are gone already, and he's in danger from at least two directions. That evening, Gwendolen writes to Chrestomanci.
Soon after, Chrestomanci himself comes in person and expresses a wish to have Gwendolen and Eric come live with him in his castle. Gwendolen is exultant, believing she's off to rule the world; Eric, who loves his home in Wolvercote, is morose. They set off a week later by train in grand style and travel to the far countryside, where they're greeted by a man named Michael Saunders, who introduces himself as their tutor. Michael drives them from the platform to Chrestomanci Castle, where he brings them through a side door and hands them off to the housekeeper, Miss Bessemer. That evening they dine with the Family and meet Chrestomanci's wife, Millie, and their children, Roger and Julia.
Next morning, Eric and Gwendolen begin lessons with Roger and Julia. Gwendolen and Julia get into a small magic duel over breakfast, after which lessons commence. Eric, who is left-handed, does as usual and pretends to be right-handed when Michael isn't looking, and Gwendolen shows she knows next to nothing about anything that isn't magic. Michael loses his temper when he discovers Eric writing with his right hand, and dismisses them both from the schoolroom before lunch so Roger and Julia can have their magic lesson: Chrestomanci, he says, has forbidden them both from learning magic for the time being.
Gwendolen, feeling she's not begin given the attention she deserves, grows steadily more and more furious and determines to make Chrestomanci notice her. On Wednesday, when they get their pocket money and a free afternoon to go down to the village, Gwendolen visits a seedy magic provisions supplier named Mr. Baslam to buy magic ingredients and some illegal dragon's blood. During dinner, she summons ghouls to loom at the windows, but Chrestomanci merely asks the butler to close the curtains.
Gwendolen declares magical war on Chrestomanci and casts one problematic spell after another as the days pass, but still no one acknowledges even that. She receives a letter from Mr. Nostrum, only to find that Chrestomanci already opened and read it, to her further fury. Her feud with Julia turns into a second war when she turns Julia's skirt to snakes during supper. On Sunday, when the go to church, Gwendolen bespells the figures and saints in the stained glass windows to run from pane to pane, causing mischief and raising a disturbance among the congregation. Chrestomanci still pays her no mind, but Millie is furious.
That Wednesday, Gwendolen's dragon's blood arrives in time for Chrestomanci's dinner party. The children eat dinner separately and are told to stay in their rooms. Gwendolen obliges and returns to her room, where she uses her dragon's blood to grow insects to monstrous sizes, summon skeletons, and conjure the same ghouls from before, and send them all to interrupt the party. Chrestomanci and Michael force their way into her room before she finishes; Chrestomanci boxes Eric's ears for not interfering, and Michael spanks Gwendolen and strips her of her magic.
Next morning, Eric wakes up and goes to Gwendolen's room, only to find that Gwendolen has disappeared and left an exact look-alike in her place. This strange girl, Janet, comes from a parallel world that has no magic and is much more modern. Terrified of what Chrestomanci will do or say when he discovers what Gwendolen has done, Eric and Janet agree to keep the switch a secret. Janet relies on Eric as she struggles to pretend to be Gwendolen, and they both must deal with the messes Gwendolen has left behind: a spell turning the maid Euphemia into a toad, twenty pounds owed to Mr. Baslam for illegal dragon's blood, an outstanding feud with Julia, and a secret plan of the Nostrums in which Gwendolen and Eric are both involved. Chrestomanci appears instantly, and immediately the Nostrums seize upon him and bind him with silver, his weakness, preventing him from using magic. They immobilize Eric and tie him to the stone before the arch, revealing that their plan with Gwendolen is to kill an innocent child--Eric--before the arch, breaking Chrestomanci's magic and opening the way to other worlds. But before they can kill Cat, Janet vanishes, and Gwendolen appears in her place. Gwendolen reveals to the Nostrums that Eric has nine lives and recounts what she did with some of them, explaining that they'll have to kill him several times; this infuriates the Nostrums, because it means Eric is a powerful enchanter in his own right, and to kill him they'll first need to discover his weakness. Before Gwendolen offers to leave so they can use her double, Chrestomanci tells them the cat, Fiddle, which Gwendolen had turned from a violin using one of Eric's lives, is in the garden. All the witches and warlocks set off through the maze-like garden to search for the cat, leaving Chrestomanci and Eric alone at the center.
Eric is shocked and heartbroken by his sister's betrayal and at how very little she cares for him. Chrestomanci galvanizes him to action, telling Eric how to use his magic to break them both free of their restraints. Eric manages to free Chrestomanci just as they're discovered, and Chrestomanci holds the returning witches and warlocks off and begins to summon the Family and Castle staff. Eric realizes Gwendolen is using his magic against them, preventing Chrestomanci from summoning Millie. He takes his magic back and joins in the battle as Millie arrives.
The Family round up the members of the conspiracy, but Gwendolen escapes and seals herself off from the rest of the world, dragging Janet back into the garden in her place.
She is a powerful witch, although it is eventually revealed that she did not have any witchcraft of her own in the first place and only borrowed magic from Eric. When the steam boat crashed, she survived because she was a witch, and could not drown. Eric had believed he had survived the flood by holding on to Gwendolen. She contained a mixture of admiration and hatred for Chrestomanci, partly because he had such powerful magic, and partly because he had supposedly not recognized her prowess.