Alan Fahey is one of the aliases that is used by a prolific serial killer who kills his victims and adopts their identities. His real name and identity is believed to be Alex Declan, who is responsible for at least eight deaths. He kills his victims, destroys any and all photographs of them and takes over their identities. In time, he grows bored and moves on to his next victim. According to Declan, when he finds the person he is meant to be he will stop killing.
When the real FBI Agent Alan Fahey found him, he killed the agent and took over his identity. The Machine produced the social security numbers of his six previous victims to point Reese and Finch towards the serial killings.
Under the guise of Agent Fahey he pretended to assist Reese in tracking down the one responsible for the serial killings but was trapped on Owen Island with a small group of people when a huge storm hit the island. He killed Deputy Erica Schmidt using Victor Engquist's knife when he thought she might have heard something about him on the police radio.
Finch had flown to Owen Island in a plane posing as a storm chaser, Mr. Gull to help Reese. But the killer saw through Finch's deception and realized that like him Finch was an impostor. He held Finch at gun point and forced him to fly him out of there. He took Finch's glasses and adapted his mannerism revealing that he will now take on Finch's identity. But Finch calls him an amateur compared to him and said that the killer could never be him because he saves lives while killer takes them and enjoys killing people.
Irritated by Finch's remark the killer tries to shoot him but Carter arrives in the nick of time and shoots him down. After recovering his glasses Finch remembers that the killer put on body armor earlier. The killer gets up and prepares to shoot them... but Cal Beecher comes up and shoots him dead.
- Previous killings
- August 10, 2005, San Francisco, California: Henri Musset
- January 31, 2007, Chicago, Illinois: George Leary
- October 19, 2009, Albuquerque, New Mexico: Roland Danvers
- April 23, 2011, Denver Colorado: Edwin Pylar
- January 17, 2012, St. Louis, Missouri: Nathan Kramer
- 2013, New York City, New York:
- February 25: Jack Rollins
- March 3: Alan Fahey
- March 6: Deputy Erica Schmidt (Stabbed with Victor Engquist's knife to prevent her from using the radio)
- March 7: Harold Finch (Attempted but failed due to Beecher and Carter)
- The title of the episode “Proteus”, referring to Greek sea-god, some call the god of "elusive sea change", who can see the future and will reveal it to any who catch him, but will change his shape to make this no easy task is probably an allusion to the killer.