Monica Reyes is an FBI special agent who was born and raised in Mexico City, Mexico, where her parents still live (as of 2002). Because she was raised in Mexico, Reyes speaks fluent Spanish. She majored in folklore and mythology at Brown University, and has a Masters degree in Religious Studies.

In 1990, Reyes enrolled to the FBI at Quantico, Virginia. Her first assignment in the FBI was serving on a special task force investigating Satanic rituals. She is a longtime friend of Agent John Doggett and serves as his replacement partner on the X-Files after the departure of Agent Dana Scully. Reyes was last seen in the New Mexico desert in 2002, where she warns Mulder and Scully of the arrival of Knowle Rohrer. She did not appear in either of the The X-Files feature films.