Diablo, originally named Noir, stands among the seven Primordial Daemons as the Primordial Black. These entities emerged from the Great Holy Spirit of darkness as counterparts to Veldanava's Seven Angels of Origin. Diablo ranks as the third-oldest Primordial, wielding power rivaling Guy Crimson, the Primordial Red. He once fought Rouge, the first Primordial, to a standstill, earning respect yet never bowing to authority. Unlike his kin pursuing power, Diablo found conventional pursuits tedious and wandered across worlds, indulging in whimsical self-amusement. Across millennia, he gathered vast knowledge and occasionally intervened in human affairs via demonic contracts fulfilled with strict integrity. His initial human encounter involved Shizue Izawa, a summoner who repelled him using an anti-magic mask. The mask's inexplicable nature intrigued Diablo, leading him to spare her and observe her from the spiritual world. When the mask shattered against Rimuru Tempest, Diablo concluded Rimuru was linked to a power transcending time, viewing him as the key to the "truth of the world." This drove Diablo to relentlessly seek a summoning contract with Rimuru. He failed during Rimuru's first Greater Demon Summoning, where his descendant Beretta answered. He succeeded during a second summoning, forcibly appearing alongside two lesser demons. Diablo pledged immediate loyalty upon summoning. He sacrificed his two demon companions to fuel Rimuru's magic, demonstrating commitment. After a period of being overlooked, Rimuru named him "Diablo," referencing the Lamborghini supercar and the Spanish word for "devil." This naming triggered Diablo's evolution from Archdaemon to Daemon Lord, altering his attire minimally from aristocratic garments to a butler outfit with a red chain symbolizing service. Later, through Rimuru's "Food Chain" ability and soul harvest during Rimuru's ascension to True Demon Lord, Diablo evolved further into a Devil Lord, joining the Twelve Guardian Lords as Rimuru's strongest subordinate. Diablo presents a complex personality: outwardly polite and calm, maintaining a relaxed demeanor with minimal emotion. He adheres to high etiquette and possesses insatiable curiosity. Beneath this lies a ruthless, calculating nature, especially towards enemies or those insulting Rimuru, provoking immediate brutal retribution. His loyalty is absolute and obsessive, stemming not from gratitude but a belief Rimuru holds existential truths. Fear of dismissal by Rimuru is his only profound terror. Diablo interprets orders literally yet exploits ambiguities to exercise judgment, as when manipulating a guard test's difficulty. Strategically, Diablo operates behind the scenes for Rimuru's goals. He orchestrated the political subjugation of the Kingdom of Falmuth by exposing corruption and eliminating leaders, while manipulating human reporters to portray Rimuru as benevolent. He recruited hundreds of demons, including fellow Primordials Testarossa, Ultima, and Carrera, forming the elite Black Numbers military force. This solidified the Jura-Tempest Federation's influence. Diablo handles diplomacy and administration, advocating governance through "gratitude rather than fear," a philosophy impressing even Guy Crimson. In combat, Diablo is Rimuru's most powerful subordinate, with an energy level (EP) of 6,666,666. He boasts near-complete immunity to physical, mental, and spiritual attacks. Key skills include "Tempter" for mind manipulation, "Great Wiseman" for advanced cognition, and "Temptation King (Azazel)" granting control over space, time, and perception. Signature techniques are "Despair Time," imprisoning foes in a nightmare realm, and "End of the World," annihilating that realm and occupants. He employs elemental and dark magic, with battle sense compensating for raw power disparities against higher-tier beings. Diablo interacts based on others' relationship with Rimuru. He respects allies like Benimaru and Ranga but harbors competitive rivalry with Shion for Rimuru's attention. He leads the Black Numbers sternly, showing occasional care for subordinates like Venom. His bond with Rimuru rests on mutual trust; Rimuru stated he would intervene personally if Diablo lost control, considered unlikely given Diablo's devotion. Diablo keeps a hidden portrait of Rimuru and enjoys serving tea, reflecting personal reverence. His role consistently centers on executing Rimuru's will through intellectual cunning, strategic foresight, and unparalleled power.