Graham Simmons was a lieutenant working at Stargate Command.
Simmons worked as a technical expert at the SGC from around 1998 to 1999, only to unexpectedly reappear once at the command and also for the final time in 2002.
Lt. Simmons assisted Captain Samantha Carter on examining the Virus Orb. When the orb opened, he was infected with the A't'trr. Dr. Janet Fraiser was able to slow the effects of the organisms with antibiotics but he was allergic to it and became very ill. Luckily, SG-1 was able to convince the A't'trr to go to P4G-881 and he survived.
Simmons had a crush on Carter. Dr. Daniel Jackson commented that he was often nervous in her presence. (SG1: "Message in a Bottle")
When Stargate Command was dialed into P3W-451, Simmons measured the gravity at the Iris and his equipment read 6 G's. Later, he evacuated to the surface. (SG1: "A Matter of Time")
When the Tok'ra sent their Sagan Institute Box through the Stargate to Earth, he organized a team to determine the radiation signature. (SG1: "Serpent's Song")
When Earth was under attack from Anubis's Stargate destroyer, he discovered the power build up in the Gate after Major Samantha Carter asked Simmons to run a diagnostic. This marked Simmons's final appearance. (SG1: "Redemption, Part 1")