Twenty-year-old Dinios “Din” Kol is the narrator and protagonist in The Tainted Cup. Din has a Signum rank in the Iudex arm of the Iyalets, meaning he is the third rank from the bottom of the officer’s hierarchy. His employment in the Iudex—the empire’s judiciary—means that he works to prevent internal corruption and crime, such as the poisonings detailed in the novel. Despite having no strong affection for his family, Din joined the Iyalets to send them money, hoping to be able to help move from the dangerous Outer Rim to the safer Inner Empire. This comparative safety comes from greater distance from the sea walls, which are at constant risk from the leviathans that approach from the sea. Din is part of the Tala ethnic group, indicating that his family comes from the outer rim canton where much of the novel takes place. Despite this origin, Din never thinks of Tala as home.

Din is a Sublime—a person whose mind has been magically altered. Din has been given the powers of an engraver, which are akin to eidetic memory: Din can remember everything he sees and hears with unerring precision. He uses these skills to aid Ana, the lead investigator in Daretana, in solving crimes.