|“||Sometimes you don't pick the instrument. The instrument picks you.||”|
— Walker in "Seek and Destroy"
Stephen Walker was a Supervisory Special Agent with the BAU, who joined the team in Season Twelve to assist in the manhunt for Peter Lewis, a.k.a. "Mr. Scratch". In Season Thirteen, Stephen was murdered by Peter in an elaborate attack on the team.
Little was known about Stephen at that moment, other than he was an experienced profiler, with about twenty years under his belt, and a member of the FBI's Behavioral Analysis Program before his transfer to the BAU. In his personal life, he was married to a woman named Monica and had two children with her, Maya and Eli. He met Emily Prentiss, then the chief of Interpol's London office, during his line of work. He was also mentored by David Rossi, taking one of his profiling classes during his first year at the Bureau. Stephen's first case concerned a terrorist cell in Belgium, and three agents were sent undercover to infiltrate it. However, Stephen's profile was wrong, and this resulted in the deaths of the undercover agents. He eventually moved on from the trauma and improved as he went along in his career. He and other BAP agents, including his longtime friend Sam Bower, were sent undercover to investigate corruption in the Russian government.
In "Scarecrow", Stephen was recruited into the BAU by Prentiss, who had become the Unit Chief of the BAU following the resignation of the Unit's previous chief, Aaron Hotchner. Prentiss had been able to expand the team's budget to allow an eighth member into the roster, and Stephen was selected for his expertise. Having admired the work of the BAU for years, Stephen accepted the offer to join the team's ranks. He is first seen in the bullpen, waiting for the team to return from a case. He greets the team as they enter, saying he wanted "to see where [they kept] the paper clips" and shares a warm exchange with Rossi. He is introduced to the rest of the BAU (except for Reid, who had taken a leave of absence for personal reasons) as the episode ends.
Immediately after the crash, Garcia contacts Matthew Simmons and they arrive at the crash site, where they discover the team's damaged SUVs. It is revealed that Peter abducted Prentiss while Stephen died from his neck snapping caused by whiplash he sustained in the collision and Garcia is extremely distraught upon discovering his body. Later in the episode, Alvez and Reid discover that Stephen had been keeping all the files regarding his investigation into Peter Lewis in Rossi's office and, per instructions from Prentiss, Stephen's files only exist as hard copies to protect against Peter hacking them. Stephen's files later proved valuable in helping Reid make progress in the hunt for Peter. After Prentiss is rescued and Peter is killed, the team, his wife, and two children attend Stephen's funeral with Monica saying that he "could rest" knowing he helped the BAU catch Mr. Scratch.
- His sidearm was a 4th-generation Glock 17.
- He played the trombone, much like his actor Damon Gupton. (Profiling 202, Unforgettable)
- He was accepted into Rossi's profiling class during his first year in the FBI, something Prentiss viewed as a particularly impressive accomplishment. (Profiling 202)
- His specialty in his work with the BAP was in counterintelligence. (Wheels Up)
- After the 2014 Isla Vista killing spree, he and his old team at the BAP investigated the Latin phrase "Et disperdam illud" ("I will destroy it") for any ties to terrorism. (Alpha Male)
- He was fluent in Russian. (Unforgettable)
- He had appeared in a total of 15 episodes as a main character, Red Light being his last although the next episode featured his corpse.
- Stephen is the first main character to be killed in the line of duty and the second to die overall, the first being Jason Gideon in Season Ten.