The fifth of the human looking Replicators found on an Asgard planet. First created him as an attempt to fix the problems Reese had suffered with emotional development, but he was considered a failure. He was the only one of the 6 Replicators that had moral objections on probing the humans minds, an opinion that the others considered to be a flaw. He was promised by Samantha Carter to be taken from the planet when the planet was to be frozen in time, but Colonel O'Neill thought otherwise. When the team left the planet, all but O'Neill had some moral issues with this action. (SG1: "Unnatural Selection")
Fifth is able to escape Hala with other Replicators, and manages to capture Major Carter, who is aboard Thor's ship. He procedes to torture Carter, by showing horrific images from her past. He refuses to show mercy, claiming that he's "not there yet". However, once he sees how much pain he is causing Carter, he relents. He then creates a dream world in Carter's mind, where he impersonates Pete Shanahan. However, Carter see through it and Fifth gives up, leaving Carter on Orilla, but not before he creates a Replicator duplicate of her. (SG1: "New Order").
Fifth then sends RepliCarter to the Milky Way, in hopes of learning about the Replicator disruptor that O'Neill created with the Ancients' knowledge. RepliCarter, however, betrays Fifth, using a disruptor satellite on his ship. (SG1: "Gemini")