Donald James Eppes, son of Alan Eppes and brother to Charlie Eppes, lives in Los Angeles. He is an FBI Special Agent, and has been the "boss" for 5 years now. Recently, he began attending therapy sessions in order to deal with the pressures of his work. Due to these same pressures, Don rarely pursues a relationship that has the potential to become serious. However, he recently picked up where he left off with Robin Brooks, an AUSA that he had dated once before.

Don is Alan and Margaret Eppes's oldest son. When he was younger, during the Vietnam War, Don attended many "sit-ins", and watched his father get arrested twice. When the young Don became fascinated with a toy gun that his uncle gave him, his parents signed him up for Little League. This became Don's second love. He graduated high school on the same day as his younger brother Charlie, a sore spot between the brothers.

He went to college on a baseball scholarship and later played for the Stockton Rangers. His backup player did steroids and went on to the major leagues, and the impressionable Don wondered if he should have to. After a critical mistake during a game, Don quit the Rangers and the next day he took the FBI test. Don will say that baseball was his first love, but his father reveals in the third season episode "Hardball" that when he and his friends would play cops and robbers, Don was always the cop.