Dexter Mayhew is a fictional character from the novel "One Day" by David Nicholls. He is introduced as a charming and ambitious young man who graduates from university and navigates life, love, and career challenges over the years. The story follows his relationship with Emma Morley, a close friend, as they meet on the same date each year. Throughout the narrative, Dexter's character experiences significant personal growth and struggles, including issues with fame and substance abuse. His journey is intertwined with themes of friendship, love, and the passage of time, making him a relatable figure in contemporary literature.