Vanir, originating from *KonoSuba* and appearing in *Isekai Quartet*, is a high-ranking demon known as the Duke of Hell and one of the Seven Great Devils. His true form is a mask, typically marked with a numeral like "II" indicating remaining lives after destruction. He presents a genderless appearance, often wearing a distinctive black-and-white mask, a formal blue suit, and white gloves. Once a general in the Demon King's army maintaining the castle's magic barrier without genuine loyalty, Vanir lost his rank after Kazuma Satou destroyed his original mask. He relocated to Axel, assisting his friend Wiz in her magic shop and operating a consultation service at the adventurer's guild. He integrates into human society, enforcing local regulations, participating in merchant activities, and earning the nickname "Crow Slayer" for pest control. Despite his demonic nature, he refrains from harming humans, viewing them as sources of nourishing negative emotions. Vanir relishes chaos, manipulating situations to provoke shame, disappointment, or embarrassment which he consumes. He exhibits a theatrical, mocking demeanor, finding humor in others' misfortunes while meticulously honoring agreements. His long-term ambition is accumulating wealth to build a dungeon filled with devils, intending to stage a dramatic defeat against adventurers. He seeks to experience the supreme negativity of their crushing disappointment upon finding fake treasure before his desired permanent demise. His abilities include precognition, emotional sensing, and limited future vision, though these falter against powerful beings like Wiz. Combat skills encompass explosive conjured dolls, a frequently misfiring "Death Beam," possession via mask attachment, curses causing body swaps, illusions, and regenerative resurrection through spare masks linked to his hell-bound core. He also employs eye beams sparingly due to self-inflicted damage. In *Isekai Quartet*, Vanir serves as a teacher for Class 1 and occasionally Class 2. He participates in school events like Field Day, summoning explosive dolls during a teachers-versus-students cavalry match. He mocks students such as Subaru Natsuki for their emotional distress and casually dismisses fellow demon Demiurge as "hard-headed," highlighting his preference for amusement over alliances. During the *Isekai Quartet* movie, he appears among school staff establishing conduct guidelines for students transported to another world. Key relationships include his friendship with Wiz, whom he aids despite frequently punishing her poor business decisions with his Death Beam. He antagonizes the goddess Aqua due to their inherent divine-demonic rivalry, engages in commerce with Kazuma Satou, and collaborates with other devils like Maxwell for schemes, underscoring his pragmatic yet mischievous role as a chaotic entity.