Fifth's name indicates that he was the fifth Human Form Replicator to be created in the image of their creator, the Android Reese.
Fifth was the fifth human form replicator created on the Asgard’s original homeworld. He was an experiment to make his programming more like Reese, their android creator, but was considered defective by the others. He appeared more compassionate and human-like than the leader, First or the other human form replicators.
When SG-1 was tortured by the human replicators, Fifth did not participate (6.12 “Unnatural Selection”). After the session was over, he carried each of them back to Prometheus because he was concerned for their comfort. He formed a rapport with Samantha Carter. He initally argued with First against SG-1 being tortured again, but relented with Carter volunteered. During this mind session, he did not invade Sam Carter’s mind, but brought her consciousness to his, so that they could speak. She convinced him to help SG-1 restart the Asgard time dilation device and stop his brethren. He was willing to help as long as they promised to take him with them. Unfortunately for Fifth, O’Neill ordered Carter to betray him, and he was trapped along with the others. His last thought before being frozen in time was disbelief that Carter broke her promise.
Somehow, a few of the replicators were able to overcome the effects of the time dilation device and escape the planet before the Asgard made the system’s sun supernova (8.01 “New Order, Part 1”). Fifth and one other unnamed humanoid were the only known human-form replicators to escape. He did not forget Sam’s betrayal and the cruelty he learned from his exposure to humanity. Fifth kidnapped Sam from the Asgard vessel Daniel Jackson. Once he had her, he sought vengeance by bombarding her mind with violent images and memories, torturing and hurting her to the breaking point.
At some point, his compassion came through again, and his mental invasion changed (8.02 “New Order, Part 2"). He took the form of her boyfriend, Pete Shanahan, and created a utopic farm in Montana where he and Sam could relax and enjoy each other’s company. Carter, however, saw through the illusion and refused to play along. Fifth revealed his true form again and explained he would never let her go because he loved her.
In the meantime, the rest of SG-1 and Thor were fighting the replicators and trying to rescue Sam once they discovered she was still alive. Fifth threatened to kill her if they did not cease their attack on his brethren, but SG-1 could not allow the replicators to escape. Fifth’s threat was an empty one, and he did not understand why they continued to fight until Carter explained that her friends knew she would never want to live trapped with Fifth. She pleaded with Fifth, saying if he loved her, he would let her go. The plea was convincing, and Fifth left her on the planet while he made his escape. However, his obsession with Samantha Carter had not ended. He was able to salvage enough neutronium to create one more human form replicator; he made it in the image of Samantha Carter.
Together with the replicator Samantha Carter, they hatched a plan to invade the Milky Way galaxy and render the technology capable of killing them inert (8.11 “Gemini”). Replicator Carter pretended to have a falling out with Fifth, betraying him and going to the real Samantha Carter with news of the impending invasion. To convince Sam of her intent, she showed her painful memories of her “training” by Fifth, but it is unknown if these were actual memories, or merely part of the ruse. Fifth kept in touch with Replicator Carter and was pleased with her progress, though worried she would be unable to sabotage the device before his arrival. Unbeknownst to him, Replicator Carter was planning a double-cross. He had taught her the lesson of betrayal he learned from the real Samantha too well but couldn’t believe his own creation would destroy him. His last word was “Why?” She explained: “You should know. You made me what I am,” just before she fired the weapon at his ship and destroyed him.