He is a slave-trader from the city of Yunkai, on the coast of Slaver's Bay.
Yezzan zo Qaggaz is a member of the House of Qaggaz and a slaver who purchases and trains pit fighters. He owns one of the lesser fighting pits. Besides the Valyrian dialect of Slaver's Bay, Yezzan also speaks the Common Tongue of Westeros.
Yezzan is present at an impromptu slave auction of Malko's. He purchases Jorah Mormont for use in the recently reopened fighting pits. When Tyrion Lannister demands to be bought as well, Yezzan is uninterested in Tyrion's claims that he and Jorah are a team. He changes his mind when Tyrion savagely beats the slaver holding his chain, and pays for him, agreeing with Malko's assertion that Tyrion is "funny". When Tyrion suggests that he and Jorah be paid for their labor to avoid the Queen's ire for practicing slavery, Yezzan hits Tyrion and tosses him a single coin. Later, at one of the "lower" pits, Yezzan addresses his fighters and announces that those who survive the day will fight at the Great Pit of Daznak. Yezzan is shocked to see that Daenerys Targaryen and Hizdahr zo Loraq themselves are present and hastily has his gladiators dedicate their kills to her. Yezzan is later knocked down by Jorah, who emerged from the dungeon early in a desperate attempt to reach the Queen.
Later, he is still at the pit observing two of his warriors when Jorah returns from Meereen. Wondering why he didn't just leave, Jorah then asks Yezzan to allow him to fight in the pit in front of the queen, and if he wins Yezzan will be allowed to sell Jorah.
At the diplomatic negotiations with Tyrion Lannister in Meereen, Yezzan returns to represent Astapor, whose slave-masters were reconstituted by Yunkai. He quickly recognizes Tyrion as the dwarf slave and remarks that he has literally climbed to the top of the Great Pyramid. The slaving cities offer Daenerys and her supporters a pot of gold to sail away from Slaver's Bay. However, Tyrion proposes a counter-offer that slavery be phased out over a seven-year period and that all slave masters be compensated. He leaves them to decide his offer. The slaving cities later attack Meereen with a large fleet
Yezzan purchased Jorah and Tyrion at auction to use in the Meereenese fighting pits. As an envoy from Astapor, he crossed paths with Tyrion again and agreed to stop funding the Sons of the Harpy in exchange for the reinstatement of slavery outside of Meereen for seven years. Yezzan and his fellow masters then violated the agreement and led an attack on Meereen. Dany spared him when she retook the city so ythat he could tell others what he witnessed from Daenerys Stormborn and her Dragons.