He is a fictional character, without historical substance, but his author intended him to show how little control mankind has on her destiny.
The year was 1747, and Voltaire also wanted to say something about orthodoxy, the established order and the rule of logic. The Book of Fate is a work of considerable influence on writers across the western world, from the Marquis de Sade to Thomas Henry Huxley.
“As Zadig was immensely rich, and had consequently Friends without Number; and as he was a Gentleman of a robust Constitution, and remarkably handsome; as he was endowed with a plentiful Share of ready and inoffensive Wit: And, in a Word, as his Heart was perfectly sincere and open, he imagined himself, in some Measure, qualified to be perfectly happy. For which Purpose he determined to marry a gay young Lady (one Semira by name) whose Beauty, Birth and Fortune, rendered her the most desirable Person in all Babylon. He had a sincere Affection for her, grounded on Honour, and Semira conceived as tender a Passion for him.”
Abstract from Zadig, or The Book of Fate, at the Project Gutenberg