Agent Seymour Simmons is a former United States government operative who serves as a key human ally in the fight against the Decepticons. His background is deeply intertwined with the secret government organization known as Sector Seven, where he served as the Chief Agent of Field Agents. His family has been involved with Sector Seven for generations, beginning with his great-grandfather, who helped excavate the first discovered Transformer, Megatron, from the Arctic. This legacy has instilled in Simmons a profound sense of duty and a personal, almost familial, stake in monitoring extraterrestrial activity on Earth.

Simmons possesses a personality that can best be described as eccentric and intense. He is known for being abrasive, arrogant, and prone to theatrical outbursts, often coming across as a jerk to those he interacts with. Beneath this gruff exterior, however, is a dedicated patriot who takes his work very seriously. He is highly intelligent and resourceful, with a suspicious and paranoid nature that makes him a skilled intelligence operative. Despite his often off-putting demeanor, he has a peculiar sense of humor and a flair for the dramatic, relishing moments where he can position himself as a crucial, lone hero, even when the situation is clearly a group effort.

Following the shutdown of Sector Seven, Simmons finds himself working unhappily in his mother's deli in New York City. He maintains his obsession with Transformers by running a conspiracy website under the alias Robo-Warrior, where he develops a rivalry with a similar site run by a student named Leo Spitz. His motivation to re-enter the conflict is reignited when Sam Witwicky, Mikaela Banes, and Leo seek him out. Although initially hostile and blaming Sam for the loss of his career, Simmons agrees to help after learning that Sam is hallucinating alien symbols. This taps into Simmons's long-held but dismissed theory that Transformers have visited Earth for millennia, and he sees an opportunity for validation and to protect his country once more. He is driven by a need to be useful and to prove that his obsession was not misguided.

In the story, Simmons plays a pivotal role as an informant and strategist. He reveals that he secretly retained a vast archive of classified Sector Seven documents and even the head of the Decepticon Frenzy. Using this cache, he provides the crucial clue that the symbols in Sam's mind are ancient Cyberglyphics found at historical sites worldwide. He then leads the group to a hidden Transformer at the Smithsonian National Air and Space Museum, which turns out to be the ancient Decepticon-turned-Autobot Jetfire. Simmons uses his intelligence and interrogation skills to help the group evade capture, leverages his contacts to decipher clues leading them to Egypt, and coordinates with the military. His most notable action is when he commandeers a radio to contact a United States Navy vessel, persuading them to use a top-secret rail gun to destroy the massive Decepticon Devastator as it climbs the Great Pyramid of Giza, thereby saving the group and enabling the final battle.

Key relationships define his journey. He initially has a hostile and adversarial relationship with Sam Witwicky, whom he blames for his unemployment. This evolves into a grudging mutual respect as they work together. He forms an unlikely partnership with Leo Spitz, the young webmaster who was his online rival, with Simmons frequently belittling him but ultimately working alongside him. He also interacts with the Autobots, particularly Bumblebee and the Twins Skids and Mudflap, often with a mixture of exasperation and reliance on their abilities. His relationship with military leaders like Major Lennox shifts from antagonistic to cooperative as he proves his value.

Simmons undergoes significant development from his first appearance. He transitions from a disgraced, bitter agent hiding from his past to a proactive, heroic figure who willingly puts himself in danger to save the world. He sheds his self-serving attitude and embraces a role as a vital part of a larger team, even if he still insists on framing his actions as those of a solitary hero. His abilities are largely non-combative but highly effective. He is an expert in espionage, surveillance, and interrogation. He is a resourceful field operative, adept at thinking on his feet, talking his way past obstacles, and using unconventional methods like tasers to achieve his objectives. His true strength lies in his accumulated knowledge of Transformer history on Earth and his relentless, if obsessive, dedication to uncovering the truth.