The character referred to as Agent Simmons in Transformers: Dark of the Moon is Seymour Simmons. He is a former agent of Sector Seven, a clandestine government organization dedicated to studying alien life, which was disbanded following the events of the first film. By the time of Dark of the Moon, Simmons is no longer a government operative but a civilian who has written a book about his past experiences with the Autobots. His family has a long and deep history with the organization, as his great-grandfather was one of the founders of Sector Seven. This legacy means his involvement with the secrets of the Transformers is a matter of personal, familial duty.

Simmons possesses a highly eccentric and abrasive personality. He is known for his paranoid demeanor, his strange habits, and his often undiplomatic way of speaking to others. Despite his odd behavior, he is a genuine patriot who takes his self-appointed mission to protect Earth very seriously. His primary motivation in Dark of the Moon is to use his unique knowledge to assist Sam Witwicky and the Autobots in stopping the Decepticon threat. Having been discredited and forced to leave government service, he is also motivated by a desire to prove his worth and be taken seriously, though his methods remain unorthodox.

In the story, Simmons serves as a crucial intelligence asset and ally to the protagonists. His past with Sector Seven has left him with a wealth of classified information that proves vital to understanding the Decepticons' plans. He operates independently of official channels, providing Sam and his friends with support that the formal military and intelligence apparatus cannot. He has a notable relationship with Charlotte Mearing, the Director of National Intelligence. Their history is a complex one; the film suggests a past romantic connection between them, and their interactions are marked by a mix of personal history, professional rivalry, and grudging respect. He also maintains his connection to Sam Witwicky, whom he views as an "alien bad news magnet" and a reliable source of information.

Simmons shows significant development from his earlier appearances. Once a powerful government agent, he experienced a fall from grace, working a mundane job before returning to action. By Dark of the Moon, he has reinvented himself as an author and a freelance expert on Cybertronian affairs. He demonstrates a pragmatic understanding of the alien conflict, correctly deducing that the Decepticons are not simply looking for something, but rather trying to hide something. His primary abilities are not physical but intellectual and resourceful. He possesses a deep, encyclopedic knowledge of the secret history of Transformers on Earth, has extensive connections from his time in Sector Seven, and is a surprisingly capable field operative who is unafraid to place himself in dangerous situations. While much of the information about his family history and younger days is detailed in supplementary materials, the film itself presents him as a seasoned, unorthodox, and ultimately reliable veteran of the secret war between Autobots and Decepticons. It should be noted that this character is from the live-action film series, and no distinct anime adaptation of Transformers: Dark of the Moon exists.