Don Tillman is a fictional character from the novel "The Rosie Project" by Graeme Simsion. He is a genetics professor in his early forties who struggles with social interactions and has a very structured, logical approach to life. Don decides to create a scientific survey to find a suitable partner, believing that love can be approached like a scientific experiment. Throughout the story, Don's life changes when he meets Rosie Jarman, a free-spirited woman who does not fit his criteria. Their unexpected relationship challenges Don's rigid views and leads him to explore the complexities of love and human connection, ultimately transforming his understanding of relationships.