John Curtis

"Zugzwang."

"The Replicator" (real identity currently unknown) is a serial killer and copycat who has been stalking the BAU at least since the start of Season Eight of Criminal Minds.

Background
Almost nothing about the stalker who has been following the BAU has been revealed, not even his or her gender (though it is more statistically probable that it is a male). However, he or she has been stalking them for some time, as indicated by the photos he or she keeps. The photos seen in the unsub's darkroom at the end of "The Silencer" suggest that the stalking began sometime after Prentiss' departure, but before Blake's joining of the team since there are pictures of the latter, but none of the former. This is further supported by his or her photo of Garcia, in which she is wearing the clothes she wore during the events of the bank robbery in "Hit" and "Run", which may be where the stalker first saw them.

The Silencer
The stalker is first seen at the end of the episode, standing in a darkroom and hanging a newly-finished photo of Blake next to photos of the other members of the team.

The Apprenticeship
After the team solves the prominent case, Hotch receives a phone call from Dallas authorities, announcing the appearance of a copycat of John Myers, a.k.a. The Silencer, who has killed a man and sewed his mouth shut post-mortem. He and Rossi discuss about it, believing that it must be a copycat since the Silencer is dead. At the end of the episode, the stalker is seen taking photos of the team during a bureau softball match from his or her car.

Magnificent Light
The stalker is next seen at a dinner that Morgan and Garcia attend. As Morgan gives a speech, he or she takes an empty wineglass at Garcia's table that was previously used by Morgan, puts it in a plastic bag, returns to his or her residence, and puts it on a table. From that same table, he retrieves a photo of his second victim, whom he killed by severing his right leg, and hangs it next to a photo of his first victim and under a set of photos of the entire team. Off-screen, he or she murders his third victim, severing his right leg and replacing it with the leg of the second victim, therefore making the stalker a serial killer.

Perennials
After the BAU conclude the latest case, Hotch and Rossi call the rest of the team into the briefing room. There, they inform them about the stalker's copycat crimes, and Hotch says that the team will be investigating this ongoing case.

Zugzwang
The stalker made a brief appearance in the episode, calling Reid on a payphone from Maeve's house. He was identified as "Adam Worth" and uttered the single word, "Zugzwang".

Broken
The stalker was seen at the end of the episode, watching live news coverage of the BAU raiding a conversion camp using illegal methods. He or she then views photos of his victims, where it is revealed he or she has already killed a woman in the style of Adam Rain. Finally, he or she views a video of Blake's lecture on linguistics and then looks at the cover of a book written by her.

Carbon Copy
The stalker will be of some significance in the upcoming episode.

Modus Operandi
During the stalker's known killings so far, he or she copied the M.O. of unsubs the BAU team have captured during the course of Season Eight. The first victim was killed the same way John Myers, a.k.a. The Silencer, killed his victims, by fatal asphyxiation and then sewing his mouth shut post-mortem; a major difference to this particular murder was that the victim was a man. The second victim lost his right leg and died of blood loss, with the third victim losing his right leg as well and having it replaced with the leg of the second victim; this was the same way killer surgeon John Nelson killed his victims. The fourth victim was killed by having her limbs dislocated, followed by a fatal hanging from a marionette device, the same way killer puppeteer Adam Rain killed most of his victims; a major difference to this particular murder was that the victim was a woman. The fifth victim, also a woman, was killed by having her head covered with plastic wrapping and then bludgeoned, the same way killing team David Turner and Toby Whitewood killed their victims; a major difference to this particular murder was that the victim was killed in Pittsburgh, Pennsylvania, while the original victims were killed in Miami, Florida. During all five murders except for the last one, the stalker went to the same cities where the original killers committed their murders. Other differences between the stalker and the serial killers he or she copies is that the stalker takes photos of his or her victims after killing them, and that in the case of the Bryan Hughes-based murder string, he or she persuaded someone else to commit the killings (of which they occurred in Philadelphia, Pennsylvania; the original murders occurred in San Francisco, California).

Personal Murders

 * Dallas, Texas: Maxwell Holmes
 * Las Cruces, New Mexico :
 * Unnamed man
 * Unnamed man
 * Phoenix, Arizona: Megan Lowenbach
 * Pittsburgh, Pennsylvania: Unnamed woman

Victims by Proxy

 * Philadelphia, Pennsylvania :
 * Molly Patton
 * Sandy Larson
 * Shannon Levin

Appearances

 * Season Eight
 * "The Silencer"
 * "The Apprenticeship"
 * "Magnificent Light"
 * "Perennials"
 * "Zugzwang"
 * "Broken"
 * "Carbon Copy"