Mainframe, alias Blaine L. Carter, is G.I. Joe's resident computer nerd. He's no pencil-necked geek, mind you - he joined the Army at seventeen, and after a brief, mind-numbing sojourn into Silicon Valley, he signed up with the Marines. Dude can probably kill you with a flash drive.
Marvel Comics continuity
After Bumblebee's destruction by G.I. Joe, Mainframe and Crankcase examined his remains at Fort Lewis in Seattle. There, they were visited by G.B. Blackrock, who informed them that Bumblebee was not the menace G.I. Joe thought him to be. Mainframe suspected that it would be possible to revive the Autobot. Mainframe did manage to resurrect Bumblebee, and partially rebuilt him. After the Joes allied themselves with the Autobots and Cobra to stop the Decepticons, Mainframe was assisted in his efforts by Ratchet and Dr. Mindbender, who rebuilt the fallen Autobot into Goldbug. Later, Mainframe, Ratchet, and Mindbender devised a way to override the Decepticon-controlled Power Station Alpha.
Devil's Due G.I. Joe vs. the Transformers comics continuity
While giving a presentation about a prototype satellite at Echelon 5, Mainframe was interrupted by Michael, who informed him of an image of one of the nascent Cobra organization's enslaved Transformers. Leaving the meeting, he left Michael in charge of completing the presentation. Now a member of G.I. Joe, Mainframe attempted to decode the message, but was unable to until the Joes' Autobot ally Wheeljack identified it as a coded message from Optimus Prime. The Autobot subsequently sent out a virus that freed all of Cobra's Transformers. Mainframe accompanied G.I. Joe on their mission to Cobra Island and battled Cobra's forces there. With the reluctant aid of Dr. Mindbender, Mainframe and Wheeljack managed to use the SPS satellite Cobra had hijacked to shoot down the nuclear missiles the US government had launched at Cobra Island, preventing a global catastrophe - all while being shot at by Storm Shadow, Firefly, and a legion of Cobra soldiers. The satellite began firing randomly, endangering everyone on the island, until Mainframe and Sci-Fi transmitted its position to Optimus Prime, allowing him to shoot it down.
Later, Mainframe attempted to defend the warpgate to Cybertron from Serpent O.R., but he was quickly struck down. After recovering, Mainframe and Doctor Li, a member of Serpent O.R.'s design team, attempted to repair the gate. When Mainframe sensed that Li was more interested in retrieving his creation than stopping its rampage, he punched the doctor's lights out. Working with Firewall and Li's colleague Martin, he managed to get it operational, and also learned the details of the Serpent O.R. project. Sending a small contingent of Joes and Autobots through the gate, Mainframe promised to send as much backup as he could find. True to his word, Mainframe sent an army of Joes to Cybertron to battle Serpent O.R. and his forces.
Some time later, while lamenting the Autobots' failure to capture Destro, Mainframe and Duke received a distress call from Firewall, alerting them to Hawk's altercation with Cobra-La in the Himalayas. As the dreaded Unicron neared the Earth, Mainframe monitored global weather disturbances, and predicted worse calamities if he continued to approach.