There is no "Elpis" series of reploids.
Going with Neo Arcadia's extreme reploid prejudice, it doesnt even name many of its governmental reploids, merely leaving them with their serial number for a name.
Its that simple. When TK31 went to the sunken library with the others and Harpuia, he accidentally stumbled upon the Elf Wars datafile, and learned of Mother Elf, aka "Project Elpis". When he was labeled Irregular for having been on the expedition, he decided to take on the name of Elpis, in his search for more power.