Cassandra Mortmain is the protagonist of the novel "I Capture the Castle" by Dodie Smith. She is a 17-year-old girl living in a crumbling castle in England during the 1930s. Cassandra is an aspiring writer who documents her life and the lives of her eccentric family in her journal. Cassandra's family includes her father, James Mortmain, a once-successful author, her stepmother, Topaz, and her younger sister, Rose. The story explores themes of love, class, and the challenges of growing up as Cassandra navigates her feelings for the wealthy Cotton brothers, Simon and Neil.