Fiona Montgomery is the main antagonist in A Cinderella Story. Fiona is based on Lady Tremaine in Disney's Cinderella (1950).
She was portrayed by Jennifer Coolidge who famously played Stifler's Mom in all the American Pie movies and the White Bitch of Gnarnia in Epic Movie.
Fiona runs a successful 50's diner called Fiona's, which used to be Hal's Diner when she married Hal, the man who runs the diner and became the legal guardian of her stepdaughter Samantha. Hal's diner where everyone is family and everybody is welcomed, Sam had everything she always wanted until Hal met Fiona and fell in love. They got married and Sam gained two step sisters. Fiona didn't show her true colors until Hal died in the Earthquake and since his father never left a will. The entire estate belongs to Fiona, including the diner and Sam.
Where Fiona treats her like a slave and make her sleep in the attic. While Sam grew up and is 16, Sam was treated like dirt and become Fiona's full-time slave without pay. Sam works really hard to graduate early and go to her dream college Princeton University like she and her father talk about.
Fiona did whatever it took to make sure Sam doesn't go to Princeton by hiding her acceptance letter and keep her as a slave forever. Things change when Sam saw the book that her father read and their was a will that came out of it and found out the entire estate belongs to Sam.
She sold Fiona and her step-sister's cars to pay for tuition and the DA is arresting to Fiona who forged her signature as a witness in her husband's will. Fiona was arrested and made a deal with the D.A. to work off the money she stole from Sam from the Diner as busting tables with her daughters. She got exactly what she deserved.