John Greer is the alias of an Englishman working for a shadowy organization that appears to be based in China, and is conducting cyber warfare.
Decima targets a man named Daniel Casey, a programmer on the run after being nearly killed by the government for learning secrets about the Machine. A Decima operative named Lambert unsuccessfully tries multiple times to recover Casey following a failed meeting, and later meets with Greer at an alleyway. Greer gives Lambert another task and warns that he will find someone else to do his job, implying that he will have him killed, if he fails.
Greer visits Kara Stanton at an unknown hospital in Dongsheng, China after she was injured in the Ordos mission. Plantilla:Crossref Stanton assumes he works for China's Ministry of State Security (MSS), but responds by joking about his tailor, indicating he has other, more lucrative employment. He tells her that she's in a sort of post-CIA "afterlife" before teasing her with the laptop in an attempt to fuel her desire for revenge. It's clear Greer wants to bring Kara over to his side. Plantilla:Crossref
On a subsequent visit, Greer attempts to connect with Stanton by discussing how the people she worked with betrayed her. He uses her study of the classics, and the mythical Titans as an analogy to motivate her to ask the one question on her mind: who was behind the attempt on her life. He offers her the chance to work for him and his organization, and in return will give her the name of the person truly responsible for her mission in Ordos, who sold the laptop in the first place. Plantilla:Crossref
Having planted the virus in the Department of Defense computers, Stanton calls Greer at the end of the mission. Greer confirms that the malware is spreading through its target network and Kara demands to know who betrayed her. Greer tells her that a name is all that he has for her because the person doesn't exist in any known database. He gives her an unheard name, which she writes down. After the car explodes, a burned piece of paper with Harold Finch written on it sits in the wreckage. Plantilla:Crossref
Now in New York, Greer, on a cell phone, talks to an unknown second person about the impact of the fall of Rylatech. He offers assurances that the U.S. government is focused on China, but unaware of his group's other activities. Greer reports that the breach of their network was the work of one person, who he will "render irrelevant", but that their larger project is on schedule.
As he does, Finch and Reese discuss Finch's tracing of code Finch saw while at Rylatech to a company called Decima Technologies, named for one of the mythological Fates who control the thread of life. Finch has determined that Decima has developed the virus Stanton uploaded with one purpose: to infect the Machine. Plantilla:Crossref
Shortly before the Decima virus counts down to midnight, Greer meets Reese and Sameen Shaw at the Thornhill offices. He explains that he knows everything about them before refusing to stop the virus, since he is invested in the outcome. He explains that Decima created the virus, but could not do so without the Machine's source code embedded within the Ordos laptop. He then reveals the name of the man that sold the laptop and the one responsible for all "this" to Reese: Harold Finch, suggesting Reese to tell Finch that Greer is looking forward into meeting him in the future. Plantilla:Crossref
One of Decima's operatives brings the drives of Samaritan to Greer. He thanks her before shooting her dead to cover his tracks, and he plans to use Samaritan for Decima's own needs. Plantilla:Crossref
Greer and Decima target Cyrus Wells, a former millionaire turned janitor who has retinal access to a room belonging to a company called Maxwell Limited - the room stores a processor chip of sufficient speed to run Samaritan. Greer and Decima arrive for Cyrus at the same time the organization Vigilance attacks, and Cyrus is saved.
Decima operatives later ambush Cyrus and Root in Central Park, and despite Reese's efforts to stop them, Cyrus is kidnapped.
After Cyrus' eye is used to bypass a retinal scanner in the room, Greer orders Cyrus killed - Reese and Fusco arrive to save him. Root arrives and in the ensuing gunfight, rescues Cyrus. The Decima operatives still take the drive and tell Greer, who is operating in a penthouse building - he tells his coworkers that their work is done, and the people burn up their computers with gasoline to eliminate any evidence. Plantilla:Crossref
Greer later hires Ken Davis, the corrupt CEO of energy company HydralCorp, to request the shipment of six generators to the USA, thus derailing their original shipment to Iraq.
At a subway terminal, Root follows Greer through a hallway, and Greer, knowing he is being followed, uses voice commands to tell Samaritan to employ counter-surveillance tactics. He places his phone in the pocket of a man resembling himself and disappears.
Root later uses Bear to follow Greer. She watches him meet with Davis, who reports that the generators have been delivered. Greer compliments Davis for his work and then has his men place a black hood over his head and take him off presumably to be killed.
Root follows Greer through the same subway terminal and this time uses Bear's searching skills to sniff out Greer, resisting his counter-surveillance measures. Greer meets her in a vacant hallway and commends her for her tracking skills. He says that it's human nature to control things and information is the new currency, offering Root a position on Decima. Root wonders why he's negotiating, and two men aim their guns at Root. Greer declares their meeting a draw and walks away, allowing her to leave. Plantilla:Crossref
Note: The virus that was uploaded by Kara Stanton on behalf of Greer affected The Machine in such a way that it failed to send the numbers of potential victims on time to Finch and led to the deaths of Alan Fahey, Cal Beecher, Bill Szymanski and Dr. Richard Nelson.
- Martin Baxter: forced to commit suicide by Greer to protect Decima's involvement Plantilla:Crossref
- Jerome Eckert: killed by a Decima sniper to protect their anonymity Plantilla:Crossref
- Bank Manager: shot twice by Greer to cover his trail Plantilla:Crossref
- Cyrus Wells: kidnapped and targeted under Greer's orders; survived Plantilla:Crossref
- Ken Davis: knocked out and presumably killed by Greer's men to cover his trail Plantilla:Crossref
- Grace Hendricks: targeted and kidnapped under Greer's orders; survived Plantilla:Crossref
- Virgil: killed himself on Greer's orders Plantilla:Crossref
- The crew of the cargo ship containing the Samaritan drives: poisoned to death to cover Decima's involvement Plantilla:Crossref
- In "2x21", we can see that the car Greer rides in is assigned a white square. It is unclear whether he still does not know about the Machine or if the box is assigned to one of the Decima operatives in the car.
- In the flashbacks in "2x13", the Machine has assigned Greer and Stanton white boxes, indicating they do not represent threats to it. By the events of "2x19", the Machine has assigned a red box to Greer, indicating he now poses a threat to the Machine.
- In "3x16", Greer's name is mentioned for the first time. The machine displays his name as "[REDACTED]" suggesting that it might be an alias. The alias is confirmed in the opening sequence of "3x17" and his title with Decima is listed as "Director, Operations".
- John Nolan, who plays Greer, is the uncle of showrunner Jonathan Nolan and his brother, film director Christopher Nolan.