Reasonably smart, practical, and often considered a brown noser, Simmons is the level-headed straight man of the Red Team.
Simmons is very self-conscious, insecure, and shy, for which reason he has not used the Red base's bathroom for three years. Simmons is also something of a know-it-all; he claims that he can mentally multiply any two large numbers, and though he gives an incorrect answer when tested, he maintains an air of confidence in his claim.
He is a compulsive sycophant towards Sarge, despite the latter's psychopathic and frequently delusional leadership. Simmons seems to regard Sarge as a father figure, despite Sarge's less-than-loving nature. By contrast, Simmons and Grif have a love-hate relationship, and spend much of their time bickering and chatting with each other. In particular, Grif often calls Simmons a "kiss-ass" due to his behavior toward Sarge. However, during their time in Valhalla in Recreation, after his experience in the simulation room had crushed his hopes and dreams, his role with Grif was reversed, as Grif was shown to be the one agreeing with Sarge, while Simmons called Grif a "kiss-ass."
Aside from supporting Sarge's flawed schemes, Simmons usually avoids foolish statements. Often exhibiting common sense, he has managed to devise battle plans that actually work. He is a brave enough warrior when necessary, but, unlike Sarge, he is not a fanatic; he has expressed an unwillingness to die for foolish reasons, although, since Grif is generally the one whose death is part of Sarge's plans, Simmons has less to worry about than he thinks.
When the Reds lost Lopez, Sarge surgically changed Simmons into a cyborg so that he would become more skilled at repairs. For some time afterward, Sarge called him "Simmons 2.0." Simmons' leftover body parts -- save for his spleen, which Sarge decided to inflate and use for "general recreation and esprit de corps" -- are later used to save Grif's life. Despite the operation, Simmons still knows very little about engineering or mechanics, mistaking a hammer for a wrench.
In Season 4, Simmons ran afoul of Sarge's fickle nature; when he insisted that Sheila was still roaming the Gulch, Sarge declared him insane and demoted him. Having lost his purpose in life, Simmons suffered a breakdown, exiled himself from the Red Team, painted his armor mostly blue, and began to wage a one-man war against his former teammates, though his plan was to humiliate them rather than kill them, at least in the beginning. He later returned to Red team; after a quick trial, Sarge imposed a hefty fine for treason and insubordination, to be split between him and Grif. Sarge seems to like Simmons even more since the rebellion, however.
In the alternate ending Where Are They Now? Simmons stole Sarge's identity after his death and attempted to assume his life. He was discovered and court-martialed by the Red Army. He is currently awaiting execution in a military prison.
In Reconstruction, Simmons and Grif were reassigned to Rat's Nest. They soon run into trouble by selling the Reds' ammunition to the Blues, and are only narrowly spared from the firing squad by the sudden appearance of Sarge. He participated in a plan to "delete" the Blue Army. Just before Simmons deleted the Blue Army, he told Sarge to rethink his decision, because there would be no reason to be Red if there were no Blues. Naturally, Sarge deleted the Blues himself while Simmons was explaining what it would be like to be Red if there was no Blue.
In Recreation, Simmons, Grif, and Sarge were reassigned again, this time to Valhalla. During this time, Grif and Sarge went to a desert after Donut appeared and spoke of a place with sand, leaving with Caboose and leaving behind Simmons, Donut and Lopez.
After this, the Meta appeared and began chasing down Simmons and Lopez. After finding the Meta's vehicle, they were caught by him and Washington, who promptly killed Lopez and Donut. Simmons was later made to call a medic who turned out to be Doc and both were held hostage soon afterward. Later, however, Simmons was rescued by Grif and Sarge and they left, leaving Doc stuck in a wall.
Simmons was later shown infiltrating a Freelancer base and, after a fight with Tex, which he and the rest of red team lost, had put the Blue team back into the database; or, more accurately, Filss put them back and began patronizing Simmons, asking "Would you like me to pretend you hacked me to make you look better in front of your friends?" After Simmons told her to stop, she sarcastically replied "Affirmative; patronizing functions deactivated" which irritated Simmons.
Simmons was later shown assisting in the fight against the Meta, helping Grif push a Warthog off of the cliff, which the Meta was attached to through a metal cable at the back.
During season 9, despite the differences of the Reds and Blues during this time compared to previously shown due to Epsilon's corrupted memory, Simmons displayed no notable differences in Epsilon's memory compared to what had actually happened.
- Simmons appears to have life issues, as he has personal problems including Grif's comment "Simmons cries in the bathroom and punches the mirror."