Missing info from: all episodes since "Beta"
John Greer is the alias of a former MI6 agent working for Decima Technologies, 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 is injured in the Ordos mission. ("Matsya Nyaya") Stanton assumes he works for China's Ministry of State Security (MSS), but he 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. ("Dead Reckoning")
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. ("Dead Reckoning")
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. ("Dead Reckoning")
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. ("Trojan Horse")
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. ("Zero Day")
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. ("Root Path (/)")
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. ("Allegiance")
in Washington D.C, Greer meets with senator Ross Garrison in front of Hieronymus Bosch’s triptych The Garden of Earthly Delights. They discuss how the machine failed to protect itself from Vigilance and how Greer can help the country. Greer tells the senator that he has an alternative to the the machine and proposes to green-light the program.
At a later time at the same location, Greer asks Garrison if he will help him or not. He then tells him that if Samaritan has access to the government feeds, it will find a terrorist for him.
Greer tells Garrison that in the time Samaritan has been active, Samaritan has found many possible targets and that a terrorist will be found soon.
Virgil tells Greer that Samaritan cannot find Harold Finch but says that he could search the name for any connections. After Samaritan finishes the search, Greer tells Virgil to bring Grace Hendricks to him.
After a failed attempt from a Decima operative to abduct Grace, Greer tells Zachary to find Grace and kill Reese and Shaw. He then tells Virgil to tell their men to position themselves wheres there is no surveillance coverage as the machine can see them.
After Grace is kidnapped, Greer interrogates her, then finds out that she was engaged with Finch four years ago. Then, Greer tells Virgil to disassemble everything as Reese and Shaw have found their location.
Shortly after Reese and Shaw arrive, seizing Virgil but not finding Grace, Greer shows up on a screen telling them that Virgil is cannot be questioned and is dead. He then tells them that he will be willing to return Grace in exchange for Finch.
After Finch is kept in an abandoned building, Greer tells him that he has been a very hard man to find. ("Beta")
In the building, Greer discusses with Finch about The Machine and Samaritan then walks away.
Then, he is in a car with Senator Garrison talking about how Decima will retain ownership of Samaritan, Garrison challenges Greer's proposal but he says that it is his terms.
Later, he is in the building with Finch telling him how Finch damaged the machine. Finch tells him that he can never understand and that it is hubris to think he could control Samaritan.
After a blackout that affects the city, Greer is reminded of his youth and talks about The Blitz. He then tells him that he wants to live under a more just rule by bringing Samaritan online.
Afterward, Greer invites Garrison in the building, saying that Vigilance abducted his colleagues and that they need to get out of the city but Vigilance breaks in and kidnaps them and Finch.
Inside a building, Greer is with the other people kidnapped by Vigilance in a Kangaroo court. ("A House Divided")
- Monica Jacobs: targeted to protect Rylatech's involvement with Decima; survived ("Trojan Horse")
- Martin Baxter: forced to commit suicide by Greer to protect Decima's involvement ("Trojan Horse")
- Jerome Eckert: killed by a Decima sniper to protect their anonymity ("Trojan Horse")
- Bank Manager: shot by Greer to cover his trail ("Aletheia")
- Cyrus Wells: kidnapped and targeted on Greer's orders; survived ("Root Path (/)")
- Ken Davis: knocked out and presumably killed by Greer's men to cover his trail ("Allegiance")
- Grace Hendricks: targeted and kidnapped on Greer's orders; survived ("Beta")
- Virgil: killed himself on Greer's orders ("Beta")
- The crew of the cargo ship containing the Samaritan drives: poisoned to death to cover Decima's involvement ("Beta")
- Harold Finch: kidnapped ("Beta") shot by Lambert on Greer's orders; survived ("Deus Ex Machina")
- Peter Collier: shot and killed by Lambert on Greer's orders ("Deus Ex Machina")
- Steven E. Soto: shot and killed by Lambert on Greer's orders ("Deus Ex Machina")
- Jefferson: shot and killed by NYPD officers under Greer's manipulation ("Deus Ex Machina")
- A number of Vigilance members, NYPD officers, journalists and civilians were killed in the post office explosion carried out on Greer's orders. ("Deus Ex Machina")
Note: The virus that was uploaded by Kara Stanton on Greer's orders 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.
- In "Zero Day", 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 "Dead Reckoning", the Machine has assigned Greer and Stanton white boxes, indicating they do not represent threats to it. By the events of "Trojan Horse", the Machine has assigned a red box to Greer, indicating he now poses a threat to the Machine.
- In "RAM", 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 "Root Path (/)" and his title with Decima is listed as "Director, Operations". In "Dead Reckoning", while talking to Stanton, Greer says, "My employers have an inkling of what happens next." It remains unknown who these employers are.
- Greer appears to be from London, England as he told Finch about experiencing German air raids when he was a child, which mainly targeted London, and that he sought shelter in the London Tube network (underground railway). ("A House Divided")
- John Nolan, who plays Greer, is the uncle of showrunner Jonathan Nolan and his brother, film director Christopher Nolan.
- "Root Path (/)"
- "Death Benefit"
- "A House Divided"
- "Deus Ex Machina"