Fifth is one of the Human-form Replicators found on the Asgard planet Hala, and later the one being responsible for creating Replicator Carter, the Human-Form Replicator doppelganger or copy of Major/Lt. Colonel Samantha Carter, up until his own death in 2004 at the hands of the Replicator disruptor satellite.
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 six Replicators that had moral objections to probing the humans' minds, an opinion that the others considered to be a flaw. He was promised by Major Samantha Carter to be taken from the planet when the planet was to be frozen in time, but Colonel Jack 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 was able to escape Hala with other Replicators using the time dilation device, and managed to capture Major Samantha Carter, who is aboard Thor's ship. He proceeded to torture Carter in revenge for her betraying him. He did so by mind-probing and then using his mind to display horrific images from her past which left Carter in tears at some point.
He refused to show mercy, claiming that he's "not there yet". However, once he saw how much pain he was causing Carter, he relented. He then created a dream world in Carter's mind, where he impersonated Pete Shanahan. However, Carter saw through it and Fifth gave up, leaving Carter on Orilla where she was eventually rescued by her team. Fifth then used the information he had collected during Carter's imprisonment to create a Replicator copy of her. (SG1: "New Order, Part 1", "New Order, Part 2").
Fifth then sent Replicator Carter to the Milky Way, in hopes of learning about the Replicator disruptor that Colonel Jack O'Neill created with the Ancient's knowledge. Replicator Carter, however, betrayed Fifth by firing a Replicator disruptor satellite aimed directly at his ship.
Seconds before the disruptor hit, Fifth wondered why, obviously hurt by the betrayal with Replicator Carter stating that Fifth should know why because he made her what she was, with the resulting shot from the satellite destroying Fifth's ship and killing him in the process.