Copy Rezo was created by the original Rezo as a test subject for experiments. The original Red Priest sought to cure his blindness and used Copy Rezo to test his theories. While Copy Rezo's eyes opened, the original's did not. Neither knew at the time that Rezo's blindness was due to Ruby Eye Shabranigdu being sealed within him. Since this wasn't the case with Copy Rezo, his blindness was cured.
Copy Rezo's sole desire was to surpass his original. Initially, he aimed to achieve this by defeating Rezo in a fair fight. However, after Rezo was killed by Lina Inverse, Gourry Gabriev, and Zelgadiss Graywords, Copy Rezo decided to either kill those who defeated Rezo or withstand the Giga Slave spell. He used Erisiel Vrumugun, the late Red Priest's assistant, to lure them to Sairaag using wanted posters. There, he tried to defeat them by resurrecting and absorbing the power of the demon beast Zanaffar, which granted him immense strength. Nevertheless, Lina, Gourry, Zelgadiss, along with Amelia Wil Tesla Saillune and Sylphiel Nels Lahda, ultimately killed him using the Blessed Blade and Flagoon. Before dying, Copy Rezo questioned his defeat. Lina explained it was due to his obsession with killing them, surpassing Rezo, and failing to envision his future. With a smile, he asked the Slayers to bury him beneath Flagoon, where he could rest in peace. Although he never surpassed the original Red Priest, he earned the respect of his enemies.
In the novel series, events unfolded differently. The original Rezo grew furious when Copy Rezo's eyes opened but his own remained closed. Out of spite, he conducted further experiments on Copy, including fusing him with a mazoku, granting Copy both pride and a will of his own, fueling his desire for revenge and to surpass Rezo. In the anime, this fusion is hinted at but its significance is unexplained.
In the manga series, events were similar to the anime, though Eris and Sylphiel's father do not appear. Copy Rezo was a fusion of a mazoku and a copy of Rezo’s body without a will of its own. He kills Eris in this version too, but this is mentioned in a flashback, noting it was when he became aware of his existence.
(Source: KanzakaDex)
