|Out of universe information|
Because his personnel file is classified, much about his life prior to 2001 never came to surface. What is known is that he was born in the United States on November 4, 1955. Before the year 2000, he was reputedly recruited into the NID by Colonel Harold Maybourne. After Maybourne disappeared from the NID in 2000, following the exposure of his unauthorized off-world operations, Simmons took over many of his old duties within the NID. He would investigate several matters concerning the Stargate Program, particularly its flagship team, SG-1. He is pretty much despised by several personnel of Stargate Command, but unknown to them, the NID and The Pentagon, he had his own agendas.
He first arrived in Stargate Command in 2001, after SG-1 found an Orbital weapon on Velona, and Major Samantha Carter saw Orlin, believing that she was delusional, he pushed the SGC to test the weapon, despite Carter's warnings that the Others would use their power to destroy the weapon, and the teams testing it, the teams went ahead anyway. After learning that Orlin was not a delusion, and was in fact real, he lead a squad to capture him at Carter's residence, only to arrive and find he disappeared through his homemade Stargate to stop the weapon's test, which he succeeded in doing. (SG1: "Ascension")
Later, while SG-1 thought they had a fifth member, a Lt. Tyler, Simmons used his access code, User 4574 to gather information on SG-1 members, and used their histories for interrogation, which he claimed were relevant to his investigations, such as analyzing Dr. Daniel Jackson's relationship to Sarah Gardner, who became host to Osiris, and his previous marriage to Sha're, the host of Amaunet- 'suggesting' that Daniel may be becoming sympathetic to the Goa'uld-, and speculating that Teal'c's brainwashing by Apophis may have simply exposed his true loyalty (Teal'c pointing out that if he was still loyal to the Goa'uld Simmons would now be dead). When he interviewed Major Samantha Carter, she became aware that he was User 4574, Simmons threatened to destroy her career because she hacked the system during his investigation of her and her team. Afterwards, he accused Major General George S. Hammond that his personal feelings were influencing his duties, and warned him that "administrations change". He then left the SGC. (SG1: "The Fifth Man")
Months later, after Major Samantha Carter was kidnapped by Adrian Conrad, because he thought she held the answers on how to remove a Symbiote after implantation, on him, Harold Maybourne confronted Colonel Jack O'Neill and claimed Simmons may have been responsible. After O'Neill's confrontation with him (after Simmons made him wait for two hours in The Pentagon), Simmons showed him evidence that he was in-fact responsible. (Maybourne was paid $3 million in his Cayman Islands account for selling a Symbiote.) After O'Neill and his team recovered Carter, he found Conrad, who was already taken host to a Goa'uld. Before O'Neill can capture him, Simmons appeared from behind and shot him twice in the arm with a silenced sidearm. Simmons offered Conrad an escape. Simmons later visited him in a cell and tells Conrad's Goa'uld that he can offer it a lot in exchange for information. He kept the Goa'uld prisoner without his superiors knowing about it. (SG1: "Desperate Measures")
With the aid of Adrian Conrad's Goa'uld, Simmons was able to contact Nirrti and form a sort-of alliance with her, Nirrti providing Simmons with the means to turn some of his chosen troops into Jaffa-level warriors in exchange for Dr. Janet Fraiser, who had played a key role in depriving Nirrti of the chance to acquire a blood sample from Cassandra Fraiser, Frasier's adopted daughter and one of the few successes from Nirrti's attempt to create a Hok'tar. However, despite Simmons's own attempt to take control of Stargate Command while Major General George S. Hammond was on a mission with SG-1 and Bra'tac to shut this program down to the extent that he ordered MSgt. Walter Harriman to keep the Iris sealed when SG-1's IDC was sent through a wormhole by claiming that SG-1 had been compromised, his efforts failed and SG-1 remained in control of the program, although unable to prove Simmons' involvement. (SG1: "Survival of the Fittest")
After Teal'c was trapped in Stargate Command's Stargate after destroying Tanith, Simmons hired Dr. Rodney McKay to find a solution with Major Samantha Carter. Simmons pressed Stargate Command for a 48 hour deadline before the Stargate was to be used again, which would effectively kill Teal'c. Simmons later privately spoke with Major General George S. Hammond and told him that he would get Teal'c out in exchange for a Kara kesh- something that could only be worthwhile if he had access to a Goa'uld. Meanwhile, Harold Maybourne and Colonel Jack O'Neill tracked Simmons' movements and found the Goa'uld in Adrian Conrad Simmons held. O'Neill informed the SGC, and Simmons was subsequently arrested and convicted for treason. (SG1: "48 Hours")
Several months later, Simmons and Conrad were released due to a hostage situation aboard the prototype United States Air Force ship Prometheus, where the hostages were Major Samantha Carter and Jonas Quinn. After he was taken on board, he worked with the hostage takers, and sent the ship into hyperspace. After Carter shorted out those controls, the Goa'uld in Conrad said Simmons was no longer any use to him, so Simmons was forced to kill him. However, the Goa'uld then took Simmons as a host, though within moments of the melding, Colonel Jack O'Neill opened the airlock, and Simmons was blown into space before the doors closed again, and died. (SG1: "Prometheus")
Sly, manipulative and cunning, Simmons fits the profile of being a sociopath in a sense that he lacks any mercy or guilt for his own actions. He also manipulates various people including Stargate Command or the NID and has his own reasons for doing so. He believes that he is doing things for the greater good but doesn't realize that his actions are possibly jeopardizing innocent people or even sending some of them to their deaths.