The protagonist of I Capture the Castle and the author of the novel’s journal entries, Cassandra Mortmain is a 17-year-old aspiring author. She begins journaling to practice speed-writing and prepare herself to become a novelist by noting her family’s conversations. She values classic British literature, particularly the novels of the Brontë sisters, and enjoys admiring the countryside near Godsend Castle. Her physical appearance is given little mention throughout the novel compared to her sister Rose, who is generally regarded as beautiful. Since graduating, Cassandra helps her stepmother Topaz with caring for the castle. She struggles to remember her deceased mother clearly and feels emotionally disconnected from her father, James Mortmain.

When she begins journaling, Cassandra hopes that she will discover a feeling of authorship within herself and learn how to write a novel. She believes herself uninterested in marriage (196) and cannot relate to her sister Rose’s desire to marry a wealthy man. Still, she helps Rose to encourage the attention of Simon Cotton. When she realizes that she loves Simon herself, Cassandra becomes increasingly depressed and disconnected from her family as she feels unable to talk openly to anyone about her feelings.