Peter is the protagonist of the novel. The story of the Hatcher family’s year in Princeton is told through his perspective. He is eleven years old and lives in Manhattan when his parents announce that he is going to have a new baby sister and, shortly after, that the family is moving to suburban New Jersey. Peter is the oldest sibling in the family and has a difficult time coming to terms with his position and new responsibilities, as well as all the changes he experiences on the brink of adolescence, within his family, and in a new city. He is an emotional and sensitive pre-teen and often has trouble containing his anger when things do not go the way he expects. Peter experiences significant growth during his year in Princeton. He goes from being highly emotional, angry, and resistant to change to adjusting to his new life, making new friends, and embracing his role as the oldest sibling in the Hatcher family. His character arc is one of maturation and acceptance.

Peter has a complicated relationship with his siblings, his parents, and his friends. Fudge requires a great deal of patience, which Peter sometimes has and sometimes does not.