Duque Duralumínio stands as the primary antagonist, a nobleman next in line for Silverland's throne after the royal family. Driven by ambition, he schemes to install his son Plastic as king to secure their family's power. His strategy targets discrediting rightful heir Sapphire by proving her biologically female—thus ineligible under the kingdom's male-only succession laws.

He frequently collaborates with subordinate Baron Nylon, who executes espionage, poisonings, rival framing, and orchestrated public challenges to undermine Sapphire. Duralumínio delegates direct confrontations to Nylon, responding to failures with verbal and physical abuse. His character merges menace with incompetence, leaning toward comedic villainy in some portrayals.

Beyond politics, he forges alliances with supernatural and militaristic foes. He aids the devil Satan in capturing Sapphire to exploit dark forces for his goals. Later, he secretly aligns with Mr. X, ruler of the proto-fascist X-Union, facilitating Silverland's invasion in exchange for power. This collaboration triggers a coup where Duralumínio captures the king and queen, intending to crown Plastic king and declare himself regent.

His relationship with Plastic reveals a sliver of humanity: despite ruthlessness, he exhibits genuine concern for his son's future, though Plastic shows no aptitude or interest in ruling. This paternal anxiety fuels Duralumínio's relentless throne pursuit.

His downfall springs from abusing allies. After the coup, Baron Nylon—driven mad by sustained mistreatment—assassinates him. Nylon attempts a power grab but is swiftly sidelined by Mr. X.

To expose Sapphire, Duralumínio relies on supernatural methods—enchanted objects, magic mirrors, and coerced confessions—reflecting the narrative's fantastical treatment of gender revelation over biological evidence.