Gwen is the daughter of Agatha Cromwell. Like her mother, she is a witch and spent her childhood in Halloweentown. At some point, she dated Kalabar, the future mayor. As an adult, she traveled to the mortal realm, where first met her future husband, William Piper, at a Halloween Party. She chose to remain in the mortal realm and live a normal life. She eventually gave birth to three children: Marnie, Dylan and Sophie. She worked to ensure that they had "normal lives", going so far as to deliberately hide their magical heritage.
Over the course of the series, Gwen becomes much more accepting of magic, and lets her children embrace their magical heritage while helping to set restraints.
The film opens with Gwen and Marnie having an argument over Halloween, which Gwen forbids them from celebrating. Dylan agrees with her, but Marnie and Sophie do not. That night, Agatha comes and regales her grandchildren with tales of Halloweentown, which they believe to be fictional. After they go to bed, Agatha has a conversation with Gwen regarding Marnie, who is due to lose her magical powers if she does not begin training soon. Gwen reiterates her desire for a normal mortal life. Agatha then reveals that the real reason for her visit is to convince Gwen to go back to Halloweentown with her. Citizens had been disappearing under mysterious circumstances, and Agatha needed Gwen's help. Gwen once again refuses, believing that there is a reasonable explanation. Saddened, Agatha returns to Halloweentown.
Unknown to either, Marnie had overheard the conversation; she and her siblings decided to stowaway to Halloweentown. When Gwen realizes this, she goes to Halloweentown herself to bring them back. However, they are told that there is no return, even though there should still be. Upon Dylan's advice, Gwen takes her children to see the mayor, whom she is surprised to learn, is her old boyfriend, Kalabar. Kalabar agrees to help them, but is called away on an apparent emergency. Out of a nearby window, Gwen notices Agatha following a young goblin and decides to follow her, as there may be trouble.
They arrive at an old movie theatre to find Agatha locked in combat with a shadow demon. The demon abruptly strikes Gwen, freezing her with a spell. Gwen remains frozen, until her children are able to complete a magical talisman that reverses the effects. Gwen and Agatha arrive at the town center to find Kalabar, the shadow demon's true identity, attempting to persuade the citizens to violently take back the mortal realm they had been forced from.
Agatha and Gwen attempt to combine their powers to defeat Kalabar, but they are only able to do so when the children help. Gwen decides to allow Marnie to begin witch training, and offers to allow Agatha to live with them. The entire family then returns to the mortal realm.
In Halloweentown II: Kalabar's Revenge, Gwen meets a handsome man named Alex, who is a single father to a son named Kal. Unknown to her, Alex is actually a golem, made of frogs, created by Kal, who is a warlock and son to Kalabar. Kal desires to avenge the death of his father, and uses "Alex" to distract Gwen.
That night, she and Alex go to the local school dance to act as chaperones. Though Dylan and Sophie soon realize the plan, they are unable to convince her until Sophie conjures a fly to distract Alex. Angered, Gwen quickly reverts Alex to a group of frogs. Gwen roams the dance looking out for Kal; she decides to wear a mask given to her to keep Kal from noticing her. However, Kal eventually returns and uses a spell to transform all the people in costume into the creatures they have dressed up as, including Gwen.
Luckily, her family is able to arrive, and combine their magic together to defeat Kal. As a result of their actions, the portal between the mortal realm and Halloweentown can now be opened at anytime; Gwen notes that her kids can travel there at anytime since they've figured out a way around the rules.
In Halloweentown High, she helps Marnie juggle nine teenage kids of different mythical heritage. Marnie had mistakenly bet the family magic that mortals and creatures could exist harmoniously. When they discover that someone is trying to steal their magic by failing Marnie's project, Gwen teleports herself to Halloweentown to find proof for the Halloweentown Council. At the same time, she works to rescue a student named Cassie, who was trapped in a witch's glass. In the end, Gwen rescues Cassie from her prison and returns to the mortal world in time to help stop the plot.
In Return to Halloweentown, Marnie is offered a scholarship to Halloweentown University. Much to Gwen's dismay, Marnie accepts; as a precaution, Gwen forces Dylan to attend as well, to keep an eye on Marnie. Meanwhile, Sophie is off doing witch training with Agatha. With the departure of her children, Gwen becomes a real estate agent and uses magic to make the houses look better.
Eventually, Gwen is summoned to Halloweentown by Marnie. Gwen learns that a group known as the Dominion wish to have Marnie use an object known as the Gift to take control of Halloweentown's citizens, and have turned Dylan into a dog to force Marnie to comply. Marnie seemingly complies with the demands; however, when Dylan is returned to normal, Gwen combines her powers with the two to "destroy" the Gift altogether.
- Though some of those affected by the Creature Spell appear to retain memory of who they are prior, Gwen doesn't appear to do so, as she scared, and even tried attacking her kids.
- She suffers from Empty nest syndrome, which is shown when her children leave in order to attend college for the first time.