Count Cagliostro, the primary antagonist of the film, serves as the regent of the small European duchy of Cagliostro. His background is steeped in a legacy of crime, as he comes from a line of counterfeiters stretching back to the Middle Ages, a family operation that has allegedly been responsible for numerous financial crises across European history. In the present, he continues this tradition by presiding over a major counterfeiting ring that produces high-quality fake currency, known as "goat bills," for nations around the world. His personal history is also connected to the protagonist, as a younger Lupin once attempted to infiltrate the Count's castle and barely escaped with his life, aided by a young girl.

In terms of personality, Count Cagliostro is portrayed as a cruel, cunning, and greedy individual. He is driven by an insatiable desire for more wealth and power, showing no regard for those he harms in pursuit of his goals. His lecherous nature is also a defining trait, most prominently displayed in his treatment of the young Lady Clarisse.

The Count's primary motivation throughout the story is to secure the lost treasure of the Cagliostro family, which would cement his power and wealth beyond measure. To unlock this treasure, he requires two ancient twin signet rings. Possessing one ring himself, he orchestrates a forced marriage with Lady Clarisse, the rightful princess, to obtain the second ring from her and legitimize his claim to the throne and its hidden fortune.

Within the narrative, Count Cagliostro acts as the direct obstacle to Lupin's goals of rescuing Clarisse and exposing the counterfeiting operation. He is the film's central villain, whose powerful and seemingly impenetrable castle serves as the primary setting for the conflict. His role is further defined by his key relationships. His most significant relationship is with Clarisse, whom he holds as a captive and seeks to marry against her will, using sedatives to ensure her compliance. He commands a loyal and deadly staff, most notably his butler Jodo, who leads a group of assassins known as "the shadows" to eliminate threats. His adversarial relationship with Lupin is the core of the plot, representing a clash between a thief with a code of honor and a tyrant with none. He also has a brief but crucial interaction with Inspector Zenigata, whom he attempts to dispose of by dropping him into the castle's dungeon, inadvertently creating a temporary alliance between his two enemies.

The Count shows no significant positive development or change of heart. Throughout the film, he remains consistently villainous, repeatedly breaking his word and betraying agreements when it serves his purposes. His story arc concludes with his demise, which occurs during the film's climax in the castle's clock tower. After double-crossing Lupin and inserting both rings into the clock face to reveal the treasure, the mechanism activates the giant clock hands, which swing rapidly toward the midnight position and crush him.

Count Cagliostro possesses several notable abilities and resources. He is a master of deception and a skilled counterfeiter, running an operation that produces nearly undetectable fake currency. His castle is a formidable fortress filled with ancient traps that have been augmented by modern technology, designed to eliminate intruders and spies. He also has access to advanced personal vehicles, such as an autogyro, which he uses in his attempts to stop Lupin. His authority as the ruler of Cagliostro gives him command over a private army of soldiers and a network of spies and assassins, all of which he deploys to protect his criminal empire and crush any opposition.