Mortimer Brewster is a fictional character from the play and film "Arsenic and Old Lace," created by Joseph Kesselring. He is a playwright living in Brooklyn, New York, who discovers that his elderly aunts are secretly poisoning lonely old men as acts of charity. Mortimer is characterized by his comedic yet anxious demeanor, often caught between his love for his aunts and the chaos that ensues from their actions. Throughout the story, he grapples with family secrets and the absurdity of his situation, leading to humorous and dramatic moments.