Simon is an antagonist and a survivor of the outbreak in AMC's The Walking Dead. He is a high-ranking member of The Saviors who serves as Negan's right-hand man and an antagonist from second half of Season 6 onwards.
Simon is shown to be a brutal, cunning and highly intelligent man. He has been shown to be extremely loyal to Negan, never questioning his orders. Like Negan, Simon isn't beyond using fear and intimidation to achieve his ends and he doesn't appear to be bothered by the excessive violence that the Saviors employ. An example of this being Negan casually asking him for a pen to mark where he would cut Carl's arm off. Simon simply assures Negan's request and shows no emotional reaction to the situation, which seems to imply that Simon has committed similar acts of brutality and has become desensitized to it. Simon also seems to be a capable leader and strategic thinker. He cleverly organized the capture and ambush of Rick's group by predicting which roads they would travel on and constructed barricades accordingly to lead them into an ambush.
Almost nothing is known regarding Simon’s life, other than he enjoyed alcoholic beverages most notably including Gin, he did however, disliked Scotch which he personally described as ‘ashtrays and window cleaner’. Simon also may had held an interest in art (as evidenced from his admiration of a masterpiece portrait of Charles V), which he accurately noted as ‘management by example’. 
At some point after the onset of the outbreak, Simon somehow came into contact with Negan and subsequently joined his group named "The Saviors", upon where the two developed a firm loyalty towards each other and thus Negan later appointed Simon as his personal second-in-command.  At some point later, The Saviors discovered an abandoned factory where together they managed to seize control over where Simon took residence inside, which Negan later established as their central headquarters as well as their personal home which became titled ‘The Sanctuary’. 
Throughout the following months, Simon aided his leader in terrorizing other survivors and communities they encountered into subjugating to Negan's will; during where he committed several brutal atrocities in order to do so, including murdering innocent people, displaying gruesome acts to implement superiority and fear to other survivors, as well as orchestrating several raids and ambushes to whoever opposed them. 
At some point after discovering the Hilltop Colony, they demanded half of their supplies in exchange for leaving the farming community unscathed, demonstrating their methods and bringing their intentions to light by executing Rory, a 16-year-old member of the colony. Roughly during the same period, The Saviors discovered another commnuity known as The Kingdom where upon meeting with the community leader they demanded a portion of supplies to be produced in exchange for leaving the commnuity unscathed.
Some time later, Simon led a group of Saviors on a hunt for Rick Grimes' people, following several attacks that left many Saviors dead. They soon discovered a small group of survivors who had taken refuge in the Viskocil Public Library; the Saviors soon terrorized the group by implementing their strict rules upon them. However, after the group retaliated, the Saviors hanged the member who first initiated the firefight inside a tower before ruthlessly executing each member of the group and allowing them to reanimate. One survivor managed to evade their attacks and escape, prompting the Savior party to chase after him.
Simon is first seen when he and his small party of Saviors hunt down a man in the woods, following an attack staged by the Saviors on the lone survivor's group. He drags the man from the woods to a road, where he and the other Saviors stand and create a roadblock, waiting for Rick Grimes' group to arrive. He stands over the captured man and taunts Rick and his crew in their RV as they pull up. Asking for their belongings on behalf of the Saviors, the leader states that negotiations cannot be made and that their proposal stands, prompting the Alexandrians to return to their RV and take an alternate route to the Hilltop. Before they leave, Rick and Simon discuss the possibilities of their days on Earth being their last, with the latter warning Rick that he should be "extra nice to those people in [the] RV", should their days on Earth be numbered.
After a number of subsequent roadblocks en-route to the Hilltop, the Alexandrians come across yet another barrier created with fallen trees. They witness the Library Survivor as he is hanged with a chain over the side of an overpass, before the roadblock is set ablaze and Simon reminds Rick to look after his people with utmost care. This display startles the Alexandrians, and they make a quiet retreat in search of another route.
Following a failed diversion by Rick's group, Simon greets them after they find themselves surrounded and outnumbered by dozens of Saviors in a small clearing in the woods. Once again demanding their belongings, he declares that the time for negotiations is now over and that the Alexandrians are simply required to listen. Simon lines the group up on their knees before commanding Dwight to present the captured search party (Glenn Rhee, Daryl Dixon, Michonne and Rosita Espinosa). All are brought together in the line-up before Simon introduces Negan to the group.
As Negan paces up and down the line-up, Simon points Rick out as the leader of the Alexandrians after he enquires. He is present for the entirety of Negan's monologue, alongside Dwight and the rest of the Saviors, and watches as Negan gruesomely beats one of Rick's group to death with Lucille.
Negan asks Simon what weapons Rick had with him. Simon reveals that Rick was carrying a Hatchet with him. Simon is seen giving Rick's hatchet to Negan after he orders him to, further proving his loyalty to his leader. Negan reveals to Rick that Simon is his right hand man, and without him, there would be a whole lot of work. Simon stays behind and watches the survivors of Rick's group as Negan grabs Rick by his shirt and drags him inside the RV.
After Negan returns with Rick in the RV and calls Carl to him Negan asks Simon for a pen to which Simon replies that he does and tosses him a pen to draw on Carl's arm. After Negan suceeds in breaking Rick and taking Daryl captive, he and the rest of the Saviors take their leave.
Simon is shown leading a group of Saviors into the Hilltop. Simon talks to Gregory in private and tells him they unleashed the walkers on The Hilltop to remind its residents that walkers are still a threat and the Saviors provide a great service by killing them. He informs Gregory that The Saviors at the outpost were killed Rick and the people of Alexandria, but Gregory pretends this is news to him. Simon says he’ll be Gregory’s new liaison and asks if there’s anything Gregory wants to tell him. “Actually, there is,” says Gregory.
He leads Simon to the foyer closet and opens it with the intention of handing over Maggie and Sasha. Instead, he finds boxes of scotch. Simon says Negan will be pleased and orders a nearby Savior to put the liquor in Negan's truck. On his way out, Simon orders Gregory to kneel, which he does with little resistance. He and the Saviors leave.
This list shows the victims Simon has killed:
It is unknown whether Simon and Negan had known each other prior or after the outbreak but they seem to have a strong relationship, as he is Negan's lieutenant and most trusted right hand man. As the second in command of the Saviors, Simon is entrusted with a considerable power and authority, close to Negan himself. He was most likely the leader in charge of orchestrating each roadblock on every possible route to the Hilltop Colony from Alexandria, preventing Rick Grimes and his group from reaching the Colony. When Rick and his group have finally been cornered and captured, he spoke on behalf of Negan by demanding them to surrender their weapons and to kneel down before introducing his leader, showing that he was given high authority.
Negan also mentions to Rick about Simon being his right-hand man and without him "there would be a whole lot of work" proving that he views highly of him.
Simon and Dwight have possibly known one another in the past. They share a neutral relationship as they are both high-ranking members of the group. However, Dwight’s brief escape from the Saviors might have affected their relationship somehow, although nothing has been seen so far. Even though both he and Dwight are Negan’s top men, Simon is shown as the secondary commander of the Saviors; his higher authority is shown when he orders Dwight to take out the rest of the captured members of Rick’s group.
Simon and Rick have an uneasy, mostly negative relationship. Initially, Rick does not think much of Simon, and considers him another minor obstacle, but as time goes by, Rick's repeated interactions with him and his men put him more on edge and cause him to become increasingly concerned for the safety and well-being of his group.
Upon meeting for the first time, Rick and his group were en-route to the Hilltop, only to be stopped Simon and a small group of his men. He demands that Rick give up their supplies, however when negotiations could not be made, Rick later informs him that they're leaving - Simon does not object and surprisingly lets them all go, showing that he at least respects his choice to leave. Both Rick and Simon share a somewhat civil discussion about the possibility of their days on Earth being their last. Simon bids them farewell and warns Rick to be "extra nice" to his group, should their days be numbered, to which Rick replies that he do the same for his people, showing that the two hold some respect for each others' views.
Later, Simon is present when Rick and his group encounter a massive roadblock constructed with fallen trees, watching as they witness the logs being set ablaze and the Saviors' captive survivor being hanged from an overpass. Reminding him of their earlier discussion, Simon taunts Rick and advises him to take extra care of his group; this warning causes Rick and the group to become uneasy and increasingly wary, and they drive hastily away.
After several roadblocks and a failed diversion which finds Rick and his group cornered by a large gathering of dozens of Saviors in a small clearing, Simon makes his presence known and taunts the Alexandrians before taking their weapons and making them kneel in a line. At this point, Rick's interactions with the Saviors have caused him to become fearful of them; he finally learns that he had severely underestimated the Saviors and is left almost speechless. Defeated, Rick attempts to negotiate before being cut off by Simon, who tells him that his time for talking is over.
Simon first meets Gregory when Simon and a group of Saviors came to take supplies from him. After Rick's people had killed every savior from one of the outpost, Simon is now responsible for the Hilltop and Gregory, being "his Negan". He tries to convince Simon about him not working with Rick, but Simon doesn't believe him. When Gregory intends to give up Maggie and Sasha to him, Instead, he finds boxes of scotch. Simon says Negan will be pleased and orders a nearby Savior to put the liquor in Negan's truck. He later forced Gregory to kneel before him.
- This character is credited as Lead Savior in "Last Day on Earth".
- In Scott Gimple's letter, read by Talking Dead host Chris Hardwick on the Season 7 Preview Special, Gimple echoes Simon's line of being "kind to one another" and credits him as Simon, revealing his name (which at the time, was unconfirmed).
- His name was officially confirmed in the Season 7 premiere.