Sven the Terrible, a vicious Viking pirate, serves as the narrative's primary antagonist. He commands the feared Frisians, pirates so terrifying even sharks flee their drakkar. Rather than raid villages directly, Sven employs cunning tactics, waiting for ships like Halvar's to gather plunder before attacking to steal their treasures, showcasing opportunism and efficiency. He typically attacks with a fleet of three ships, complicating direct confrontations.

Sven wields a morning star on a chain with notable proficiency, throwing it easily in battle. Accounts, like Urobe's, describe his extreme childhood ferocity, viciousness, and cunning. Universally hated, he burns villages and strikes down anyone obstructing him without hesitation. He constantly seeks conflict but is repeatedly thwarted by the protagonist Vicke's clever, non-violent strategies, leading to Sven's frustration and desperation. One instance involves him stealing a king's scepter; the king initially misidentified Sven due to poor eyesight, but upon receiving glasses, correctly identified him, leading to Sven's defeat. In later film adaptations centered on a magic sword, Sven is depicted as the sword's original owner before Halvar steals it, reinforcing his adversarial role.

Sven rules through intimidation. His second-in-command, Pox, outwardly flatters him as "Captain" and "the most feared man across the North Seas" but inwardly resents and fears him. Across all official media, Sven remains an unrelenting antagonist without significant evolution or redemption, embodying piracy, greed, and brute force as a counterpoint to the protagonist's intellect and pacifism.