Timothy
Timothy getting a pnuemonia early in the book is a big curve in how Mrs. Frisby and her children planned to live their lives. Mrs. Frisby would have never met the Rats of NIMH or get captured by Billy. The Frisby's would have just moved to their summer home, continued living their lives as it was planned to be. Timothy also stayed back when Cynthia was sick and helped her recover to full health.