James Mortmain is a fictional character from the novel "I Capture the Castle" by Dodie Smith. He is the father of the protagonist, Cassandra Mortmain, and is portrayed as a reclusive and eccentric writer who struggles with his career and personal life. Living in a crumbling castle, he is often lost in his thoughts and writing, which affects his relationship with his family. Despite his literary aspirations, James faces financial difficulties and is largely dependent on the income from the family estate. His character reflects themes of creativity, isolation, and the challenges of artistic ambition, making him a central figure in the story's exploration of family dynamics and societal expectations.