Diablo, originally named Noir, ranks among the seven primordial daemons as the Primordial Black. As the third-oldest primordial, he existed millennia in the spiritual world with whimsical tendencies and disinterest in conventional power pursuits, often irritating fellow primordials. His exceptional power efficiency trivialized conflicts, allowing him to remain an archdaemon despite strength sufficient for daemon lord evolution. A pivotal shift occurred when he answered the Silver Wings adventurers' summoning, confronting Shizue Izawa. Her anti-magic mask repelled him, sparking fascination that led him to secretly observe her. After the mask broke in Rimuru Tempest's possession, Noir concluded Rimuru held the key to the "truth of the world" and sought to serve him. Noir attempted to answer Rimuru's greater daemon summoning within the dungeon but was thwarted by his descendant Beretta. He succeeded during a subsequent summoning targeting the sorcerer Razen, arriving with two greater daemon followers. After proving his value, Rimuru named him "Diablo"—referencing the Lamborghini Diablo supercar and Spanish word for "devil"—triggering his evolution into a daemon lord. Diablo pledged absolute loyalty to Rimuru, serving as personal butler and second secretary in the Jura-Tempest Federation. His devotion manifests as profound fear of dismissal and ruthless punishment toward Rimuru's insulters unless countermanded by direct orders. He interprets commands literally but exploits ambiguities to act according to his judgment, often prioritizing proximity to Rimuru. Demonstrating loyalty, he recruited hundreds of daemons—including the three remaining primordials—to form the elite "Black Numbers" military unit he commands. His personality blends calculated whimsy with high etiquette. Outwardly relaxed and emotionless, he displays curiosity and desire for new experiences, masking cold ruthlessness toward enemies while treating Rimuru's allies amiably. He intensely avoids indebtedness, exemplified by secretly saving Shizue's student Tiss from a phantom and demanding secrecy as repayment for Shizue's earlier impact. Strategically analytical and individualistic, he prioritizes efficiency and long-term objectives, frequently employing deception. His motivations fuse intellectual pursuit with a need for validation through service. Diablo appears humanoid with black hair streaked red and gold, golden eyes featuring red pupils and black sclerae, and concealed horns and claws. As an archdaemon, he wore aristocratic attire. Evolution into a daemon lord altered his wardrobe to a butler outfit with a red chain symbolizing service, amplifying his dignity while retaining his association with black. In combat, Diablo is Rimuru's strongest subordinate among the Twelve Guardian Lords. His strength stems from battle sense, techniques, and extensive magical knowledge rather than raw power alone. Though inferior to Rimuru—especially post-demon lord evolution and the "Food Chain" power-sharing ability—his prowess remains formidable. Elmesia, the Fairy Queen, expressed theoretical concern about his potential to berserk, though loyalty renders this improbable.