Don is an FBI Special Agent who recruits his mathematical genius brother, Charlie Eppes, who is five years his junior, to help him and the Bureau solve some of their most difficult cases. Don makes great sacrifices in his personal life in order to devote himself to his career, which to him is more of a way of life than a mere job. More important to him than his work is his family, especially Charlie, even though he doesn't understand his brother's way of looking at the world.

Don gave up his position as Special Agent in Charge of the Albuquerque FBI office to move back to L.A. when his mother became ill with cancer. He doesn't understand how his brother could lock himself up in the garage with a possibly unsolvable math problem, P versus NP, for the three months before their mother died, never going to see her. It's a point of contention between them (1st season - "Uncertainty Principle" and a brief mention in "Vector"). After Charlie found a classical music composition for the piano (an Etude in G minor) that their mother wrote under her maiden name and his father admits that music was one of Margaret's first loves, Don and his brother remember the piano lessons that she made them take. While Don jokes about the dreadful teacher they had, he and Charlie realize why it was so important to their mother. At the end of "Running Man" (2nd season), Don comes home and finds the sheet music his mother wrote on the table. He sits down and starts playing, hesitantly at first, as Charlie and Alan listen from the garage.

In "Provenance," Don began to wonder why their family was not more religious and wanted to get in touch with his relatives, especially his grandmother's cousin who escaped World War II. A victim of the Holocaust found solace when Don had given a stolen painting which held much sentimentality back to her. Prior to this she had said that he knew pain in losing his mother. All of this had surfaced deep emotions in Don. Also in this episode, Don has jokingly stated he was a "lost cause," when his father expressed a failure in parenting him. On the other hand, in the previous season's "Calculated Risk," Don showed a connection to a boy who had lost his mother, giving hope to Alan that he had done something right.

When Megan Reeves was kidnapped, Don pushed his ethics to the limit to get her back. While he did not beat a suspect, he had another torture the man. Don showed concern to his father. After this discussion, he convinced his father that it is good to have someone to come home to, and that is why he comes around the Craftsman house so often rather than staying at his apartment. He is a harsh taskmaster and is not very forgiving. One of his former tactical trainees from Quantico, Liz Warner, has proclaimed to this and, after Colby confided in her that he made a mistake, she told him that the very fact that he still remains on Don's team means something. She also said that Don never spoke of his personal life while at the FBI Academy and was at the time fresh from the field in hunting fugitives (with friend Agent Cooper). Liz knows he has mellowed with age and he can hold a commitment with a woman for longer periods of time. (Courtesy of Wikipedia.)