Don recruits his mathematical genius brother, Charlie Eppes, to help him and the Bureau solve some of their most difficult cases.

The day after realizing that he would never be better than a single A player, Don quit the Rangers and signed up for the FBI entrance exam ("Identity Crisis").

After graduating from the Academy, Don worked in Fugitive Recovery with Billy Cooper (episode "Man Hunt") and former Navy SEAL Petey Fox ("Friendly Fire") and was very good at it.

After that, he worked in the field office in Albuquerque, New Mexico, and dated Kim Hall (Sarah Wayne Callies), and nearly married her.

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, from which she eventually died.

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.

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).

[2] 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.

His relationship with Charlie was apparently somewhat strained even before their mother's death, as Don is stunned to learn that his little brother has enough security clearance to work with both the CDC and the NSA ("Vector").

He is even more surprised to discover that Charlie doesn't seem to know that his clearance will allow him to walk into the L.A. Office whenever he wishes, rather than obtaining a visitor's pass every time and being restricted in what he can do there.

It is never shown or said what Edgerton did during the interrogation, but it is heavily implied that he may have used physical force, up to and including applying pressure to the gunshot wound through the teen's wrist; Don expressed concern to his father over being willing to go that far, even if he wasn't doing it himself.

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.

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.

In "Blackout," after Robin broke up with him, Don became involved in a brief romance with Special Agent Liz Warner, whom he remembers for having an issue with adrenaline and being wild.

Further, Alan recalled how Don would take on others' burdens as a child and told him to get counseling, which he later willingly sought, especially considering that he viewed his life's work as a series of dirty deeds.

This shows how far the relationship between the brothers has come, especially since being damaged by the unusual pressures of their childhood and the three months near the end of their mother's life that Charlie spent working on P vs. NP.

In "The Janus List", Don sympathizes with the brilliant cryptographer's desire to feel that his life had meaning, showing another side of him to his father, as he usually would not appreciate any sentiments of a criminal.

While examining the position of cigarette butts on the scene, he suddenly drew several connections and determined the suspect's real motives for breaking his apparent pattern.

2) During "The Janus List" (Season 3 Finale), Don is looking at pictures of the placement of explosive devices on a bridge in the L.A. office with Charlie.

This pattern, though something he initially shrugged off as a coincidence, turned out to be part of the key to revealing the Janus list and Colby Granger's position as a spy for the Chinese government.

In the episode "In Security," Don hides the level of intimacy he had with Leah Wexford and information contained in cleared FBI files therein concerned, after she is killed.

Don feels overwhelmingly guilty, and so Charlie performs a Classification And Regression Tree (CART) analysis behind his back.

In the season finale, he is upset that Charlie cannot help him anymore due to his extreme act of sending his friend's work to a terrorist.

When season five opens, Don asks Amita and Larry to help with the math since Charlie no longer had his clearance, which neither brother wants to fight for.

The fight with McGowan, coupled with seeing a fellow agent reach out for a picture of his family before dying in a shootout, causes Don to explore Judaism.

After Charlie tells him that he cannot go back in time and reverse his actions, Don then makes a plan to capture Buck alive.

At home, Don confides in Alan that he had his faith in God's existence shaken by the stabbing and that he does not know whether he wants to continue his spiritual journey.

Although, he admits he has a "lost a step" to his father, not remembering when he has beaten David or Colby in a sprint, he is quick enough to kill his old, tough FBI pal Pete "Petey" Fox, who is an expert in a gunfight and has fallen to the wrong side of the law.

[6] To prepare for his role as Don Eppes, Morrow did some training with the FBI as well as reading about and talking to real-life police officers.