Sophie is a half-human witch. She is the daughter of Gwen Piper and the granddaughter of Agatha Cromwell. Additionally, she has two older siblings, Marnie and Dylan. Gwen, wanting her children to live a normal life, initially made deliberate efforts to hide Sophie's developing powers, before coming to terms with them. Sophie is shown to be resilient and quick at problem solving.
As a descendent of the Cromwell family of witches, Sophie has potentially powerful magical abilities. Early on, she was able to trigger magical effects unknowingly. While she is still being trained in the use of her powers, she is able to cast simple spells and appears to be better than remembering them than Marnie. Sophie has strong precognitive abilities and the ability to sense the presence of "bad things".
Sophie grew up in the mortal realm, living a normal life with her mother and siblings. Gwen never allowed Sophie to do any Halloween-related activities, and though she is beginning to develop magical abilities, Gwen intentionally hides them from her. That night, her grandmother, Aggie, arrives, and begins encouraging her grandchildren in Halloween-related activities.
That night, Sophie overhears Marnie and Dylan talking about a conversation between Aggie and Gwen, which revealed that both were witches and that Marnie was as well. When Marnie and Dylan follow Aggie to the world of Halloweentown, where Aggie and Gwen are originally from, Sophie secretly follows them. After an encounter with Kalabar, the mayor, the kids arrive at Aggie's house. While the gate is locked, Sophie unknowingly uses magic to unlock it, by wishing that the lock was a frog. Aggie decides not to take the kids home right away, and promises to train Marnie after dealing with a problem, which she shows to the children in a cauldron. Aggie explains that she is attempting to light a talisman that will fix the problem, but the spell fails due to the use of instant witch's brew.
Gwen eventually arrives to take her children home, but they find that they are trapped in Halloweentown. Gwen goes to the mayor for help, but he is forced to leave abruptly. Noticing Aggie following a goblin named Luke, they believe her to be in danger and follow her to an abandoned movie theater. A demonic creature arrives and freezes Gwen and Aggie, though the kids escape. Deciding to finish the spell that Aggie had been working on, Sophie helps her siblings collect the ingredients. However, Marnie is unable to remember the spell. Sophie does, luckily, and so she helps her cast it successfully.
The kids realize that they must install the talisman in the jack-o-lantern in the center of town. By this time, the demon has arrives and reveals himself to be Kalabar, who wants to take back the mortal realm. While Sophie remains hidden with Dylan, Marnie is able to sneak over to the jack-o-lantern after a distraction by Luke. Though Marnie nearly passes out after being hit by Kalabar's spells, she manages to drop the talisman into place, which unfreezes Aggie and Gwen.
Sophie would later combine her powers with Aggie, Gwen, Marnie, and Dylan, who manifests powers out of anger. Together they are able to defeat Kalabar. Sophie then returns home with her family, Aggie included.
Sophie does not appear in Return to Halloweentown. The film states that she is traveling through worlds as part of her witch training under Aggie.