An FBI Special Agent who runs the Bureau's (fictional) Violent Crimes Squad in Los Angeles, Don often drags his mathematical genius brother, Charlie Eppes, in 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.
Even 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, or how he knows a fraction of the things he does.
A successful and highly decorated FBI Agent, Don spent time as an instructor at Quantico, and as a tracker, hunting fugitives. These exciting and often solitary activities kept him apart from his family.
Before the death of his mother, he had worked in Alberquerque in a very high position in the FBI. As his mother died, he decided to go back to work at the Los Angeles FBI to be again closer to his brother and dad. This is also the point where the series starts.
Don is a principled character and very devoted to his job, leaving him not much time for a social life. According to actor Rob Morrow, Don has a tendency to sacrifice, even at the expense of what he wants from life. He enjoys the occasional game of baseball and is also often seen watching hockey, which he played in his youth. He went to college on a baseball scholarship. Don thought baseball was his first love, though his father Alan Eppes reminded him that a toy gun and playing a cop as a child was actually his first love, and comforted him in noticing that the FBI needs utility players. Charlie said that he was a "born cop". Don's giving up on the game is an emotional subject for him.
He and Charlie have had their differences over the years, and Don finds it hard to go to Charlie for help to do his job. In fact, in "One Hour" Don states that he does not like living in the shadow of his genius brother, though he respects him and his abilities very much and they have become closer. Sometimes, he feels that Charlie isn't doing all the great things he could be doing because he is working with the FBI. Still, he does often question his brother only to have his faith in Charlie's abilities restored. In "Burn Rate", their father says, "But have you ever known your brother to let his emotions trump his math?" Just because Don gets emotional, he thought this was the case with Charlie and even stated his brother was backing one of his own. In "Trust Metric", Charlie says that he has a giant ego.
He is a harsh taskmaster, likes being the boss, and is not very forgiving. One of his former tactical trainees from Quantico, Liz Warner has testified 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 meant 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. He believes that the death penalty is a form of revenge.
Don has an ability to understand how criminals think, which causes him to suppress his thoughts in an effort to cope with the horrors that he sees on his job.
- Don's main weapon of choice is a Glock 19.