A blind but extremely powerful, known for his good deeds...usually.
Rezo is a blind priest known as Rezo the Red Priest who is famous for his wisdom and good deeds. He is at least a century old, although he doesn't look it, and Zelgadis is his only known relative. Unfortunately, Rezo was born with one of seven shards of the demon Ruby Eye Shabranigdo sealed within his eyes, causing his blindness to be uncurable by magic, and later in life, corrupting his soul-- leading to any number of less-than-good deeds, including cursing Zelgadis.
In this corrupted state, he came to the belief that if he revived Shabranigdo, the demon would grant his sight. Fortunately Lina was able to kill the demon using the Giga Slave spell and Gourry's Sword of Light and Rezo's own mournful soul, although Rezo passed away in the process.
However, a second Rezo appeared that turned out to be a clone of Rezo, created so that Rezo could experiment on curing his blindness. The clone desires nothing more than to surpass the original, ideally by killing him in battle. When Rezo died with Shabranigdo, the clone decided instead to defeat Lina and her group. In the anime, he absorbed the power of the demon beast Zanaffar in order to gain the power to defeat them; in the novels, his power is the result of additional experiments of the original Rezo's. Either way, Lina and the team were able to destroy him.
Since in the anime and novels we mostly only see the corrupted Rezo or his clone, it's hard to say exactly what Rezo the Red Priest is truly like. From all reports, however, he is kind, helpful, and wise.