This article is for the general definition of what a Reyvaroid is. For information on the boss of the same name, please go to the Reyvaroid (boss) article.
A Reyvaroid (レーヴァロイド, reevaroido?, lit. "Revaroid") is the supposed next step in human evolution created by the Archia Think Tank, more specifically, the Slave race of the new humans, which completely lack egos and wills and are instead controlled by the crystals embedded in the bodies of the Ruler race of the new humans; and are the final result of the Human Evolution Project. These creatures are a mix between humans, Antibodies and Reyvateils, although strictly speaking, the only true part of the mix are the humans and Antibodies, while the Reyvateils are used as a sort of "glue" between both forms.
This is because the Archia Think Tank needs to draw away their SH Server Address and give them to the future Reyvaroids in order to execute their transformation, although this will end up killing the Reyvateils involved due to severing their connection to the SH Server. This is also the same reason why Archia also allowed the Reyvateils to prosper: they needed to have as many SH Server Addresses registered as possible, as otherwise the Reyvaroids would have developed the Border Disease and the project would have ended in failure. This is also why Aisha, the younger sister of Hikari Gojo, and many other children died after they were given the Blocking Operation: because their SH Addresses were stolen and manipulated for testing out the human-Antibody fusion, causing the complete collapse of their minds.
The only Reyvaroid featured in Ar tonelico Qoga: Knell of Ar Ciel is Gengai, who was transformed into one after falling in Raphael's trap. However, since the fusion hadn't been completed at the time, the party could return him to normal by defeating him in battle, although that horrible transformation would have been the fate of everyone in Sol Cluster if Richaryosha had managed to sing EXEC_Z/. to the end.