Copy Rezo was created by the original Rezo as a lab rat for experiments. The original Red Priest was seeking to cure his blindness, and used Copy Rezo to test his experiments on. However, while Copy Rezo's eyes opened, the original's did not (though neither knew this at the time, the reason behind Rezo's blindness was that the dark lord Ruby Eye Shabranigdu was sealed within Rezo. Since this wasn't the case with Copy Rezo, his blindness was cured).

Copy Rezo's greatest and only wish was to surpass his original. At first, he intended to achieve this goal by killing Rezo in a fair fight. However, when Rezo was killed by Lina Inverse, Gourry Gabriev, and Zelgadiss Graywords, he decided that the only other way to surpass Rezo was either to kill the ones who defeated the original Rezo (namely Lina, Gourry and Zelgadiss) or withstand the Giga Slave. He used Erisiel Vrumugun, the assistant of the late Red Priest, to lure the three of them to Sairaag with wanted posters. There, he attempted to defeat them by resurrecting and absorbing the power of the demon beast Zanaffar (which gave him an enormous amount of power). However, Lina, Gourry, and Zelgadiss (along with Amelia Wil Tesla Saillune and Sylphiel Nels Lahda) killed him using the Blessed Blade and Flagoon. Shortly before his death, Copy Rezo wondered how he had lost. Lina answered that his desire to kill them, surpass his original, and his inability to see what he could have done in the future afterwards was the reason for his failure. Then with a smile on his face, he asked the Slayers to bury him beneath Flagoon, where he could rest in peace, and even though he never surpassed the original red priest, he had won his own enemies' respect.

In the novel series, things happened a little differently. The original Rezo became enraged when Copy Rezo's eyes opened but his own would not. Out of spite, he conducted a number of other experiments on Copy, one of which included fusing him with a mazoku (as opposed to Copy absorbing the power of Zanaffar, as it happened in the anime). Becoming a half-demon gave Copy Rezo both pride and a will of his own, causing his desire to get revenge on and surpass Rezo. In the anime, the fusion with a mazoku is also hinted at, but the event's significance is never explained.

In the manga series, things happened similarly to the anime, but Eris does not appear and neither does Sylphiel's father. He was a fusion of a mazoku and a copy of Rezo's body which did not have a will of its own. He kills Eris in this series too, but that is mentioned as a flashback, were he says that it was then when he became aware of his existence.

(Source: KanzakaDex)