Jeff Simmons

was the person previously known as the Quantico Gamma. We learned his name in Act IV of episode 3x07 "Ligature":


 * "So somebody killed Viv Paliotto. And somebody killed Joshua Lynch. And I've had some other dreams about dead people, too, where the only ligature strangulation was in the dream--remember Jeff Simmons?"


 * Reyes's arch expression made it pretty plain he considered it pretty unlikely that anybody could forget chasing a gamma through Hogan's Alley at Quantico.

He was a former cop and FBI cadet.