Cassie Andrews is the protagonist of The Book of Doors and its primary point-of-view character. She’s characterized by her love of books, something that those around her understand to be a cornerstone of her actions and perception of the world. When she first arrives in New York, preceding the events of the novel, her immediate safe place is an independent bookshop. She later ends up working there and meeting her best friends, Izzy and Mr. Webber. The bookshop and what it represents in Cassie’s life is the foundation of her adventures in the novel.

At the beginning of the novel, Cassie is idealistic and selfish, exposing a youthful immaturity. When she receives the Book of Doors as a gift from whom she understands to be a casual acquaintance, she sees only its potential for fabulist adventures. She also exhibits this same selfish nature when she leaves a cafe without paying and disregards Drummond’s concerns, and when she becomes defensive and resistant to Izzy’s critiques of the Book of Doors. While Cassie’s choices are not malicious in any way, they are narrow and naïve; she takes the path of least resistance to the experience she most wants to embody.