Stephen Walker

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"

Background
Little is known about Stephen at this 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 is married to a woman named Monica and has 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.

Season Twelve
In "Scarecrow", Stephen is 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.