When the Replicators found Reese, they hailed her as their creator, and found that she possessed certain advantages over the standard insect-form Replicators. They used her nanite cell technology to create First, to serve as their leader. His abilities enabled him to deactivate the Asgard time dilation device.
Fifth, however, allowed SG-1 to activate the device, and First revealed that they had made a fool of his "son". First was presumably destroyed when Hala was shattered when the Asgard collapsed its sun into a blackhole. (SG1: "Unnatural Selection", "New Order, Part 1")