According to the writer of the episode, David Amann, executive producer Frank Spotnitz was interested in giving Reyes "some darkness to play" and her past life was consequently written with sombre overtones.

[9] Reyes then served in the New York City field office, where she became romantically involved with special agent Brad Follmer despite the FBI's strict anti-fraternization rules.

Following a year of investigating the paranormal, Doggett and Reyes were last seen in the New Mexico desert in 2002, where they were warning Agents Mulder and Scully of the arrival of Knowle Rohrer, a Super Soldier linked to the alien colonists.

The X-Files office was closed shortly after the involvement of Walter Skinner and Alvin Kersh in Mulder's escape was revealed;[13] Reyes still stayed with the FBI in some capacity for a time afterwards.

Shortly after the closure of the X-Files, Reyes was contacted by the Smoking Man[14][15] who had somehow survived his apparent death in "The Truth", albeit at the cost of suffering devastating injuries.

In exchange for her assistance, he offered her a place amongst the survivors of the End Times, who would be spared from the effects of the "Spartan" virus, administered to the population via smallpox vaccinations.

Following multiple outbreaks nationwide linked to the Spartan virus, Reyes contacted Dana Scully in early 2016, willing to share the knowledge needed to develop an appropriate vaccine.

In the eleventh-season premiere, it is revealed that despite the events of the season ten finale "My Struggle II" being a part of Scully's vision, Monica is still working for The Smoking Man as one of his confidants and as a member of the new Syndicate.

In the season finale, "My Struggle IV", she contacts Mulder and Scully with information pertaining to their son, William, revealing that she is working from the inside to keep them safe.

[17] Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen, felt that Reyes' introduction in "This Is Not Happening" was "rather forced", finding her upbeat personality at odds with the tone of the series at that time; Shearman and Pearson also felt that the overall use of the character in season nine was "lazy", with her willingness to believe in anything compromising the tension of episodes such as "4-D" or "Hellbound".

[20] Writing for The New York Times, Joyce Millman described Reyes and her partner Doggett as "the Diet Coke of Mulder and Scully", referring to their secondary standing.