Sophie Amundsen is the main character in the novel "Sophie's World" by Jostein Gaarder. She is a curious and thoughtful teenager who begins to explore philosophical questions about life, existence, and knowledge. Throughout the story, Sophie receives mysterious letters that introduce her to various philosophical ideas and thinkers. As Sophie delves deeper into philosophy, she learns about important figures such as Socrates, Plato, and Descartes. Her journey not only expands her understanding of philosophy but also leads her to question her own reality and identity. The novel combines elements of fiction and philosophy, making it an engaging read for those interested in these topics.