Diablo

""Hee-hee-hee-hee-hee. The birth of a new Demon Lord! Quite an old sensation but a familiar one. What a truly wonderful day ! Such a offering-and the very first order from our Lord, no less. This is such an honor; I couldn't possibly be more enthusiastic about this. Would it be all right to continue serving you in the future?"."

- Diablo

Diablo (ディアブロ), also known as Noir (原初の黒) is one of the seven Primordial Demons and an Executive of the Jura Tempest Federation as the Second Secretary of Rimuru Tempest.

Arch Demon
Black hair with streaks of red and gold, as well as golden eyes, Noir was finely dressed in garments like that of a prince. Other than that he looked like a normal human, though he was able to unfold wings as well.

Demon Peer
Diablo's change in appearance is possibly the least drastic after any evolution, only his princely garments were replaced by a Butler outfit, reflecting his wish to serve Rimuru, but despite that, his dignity didn't decrease, but rather even increased.

Personality
Diablo is whimsical but also very calculative when he wants to be. He usually shows an absolutely composed attitude, but whenever anyone insults Rimuru, his polite exterior turns into a cold form of malice and he proceeds to punish the "sinner" in the most painful way imaginable unless it goes against Rimuru's orders. However, when someone has a good relationship with Rimuru, Diablo usually treats them in an amiable way, though he might still manipulate them for his own ends, with "his own ends" usually referring to being close to Rimuru more often.

Diablo follows Rimuru's orders to the letter. When tasked with increasing the difficulty of Framea's test as a guard for guests to Tempest, Diablo summoned a moderately strong monster. The moment Framea got paralyzed by the creature's goo and still showed a will to fight, Diablo sent the creature away and went to report back on Framea's actions. Should there be some "grey area" in his instructions, Diablo has the freedom to carry out his orders how he sees fit.

The greatest show of Diablo's loyalty to Rimuru was recruiting hundreds of other demons (including the last three Primordials) to serve his master; the creation of the Black Numbers, who became the de facto strongest fighting force in Tempest.

Diablo also despises owing a debt, even to someone who has already passed on. Shizu's replacement, Tiss did a desperate summon to save Shizu's Students from a Phantom, only for the unnamed Diablo to practically give his aid for a pittance of a price by forbidding her from mentioning his involvement, and proceeded to slaughter the aggressor in a truly horrific fashion, thus fulfilling his debt.

Background
Thought of as strange by the other Primordials, Noir only acted on whims and didn't show any regular behavior, to the irritation of some of the other Primordials. Due to that he one day stopped pursuing power as his power efficiency became so high that any fights were too easy for him, causing him to remain as an Arch Demon despite having enough power to evolve into a Demon Peer already.

One day when he personally answered a Demon Summon of a Silver Wing on a whim, he had a chance encounter with Shizue Izawa. During a confrontation, where Noir was about to finish her off, he was repelled by Shizue's Anti-Magic Mask instead, making Noir realize that the mask has the power to compress time; power of infinity. After those events, he returned to the Spiritual World and continued observing Shizue and the mask in secret. When he saw that the mask broke and lost its power when the mask came into Rimuru's hands, Noir came to the conclusion that Rimuru is connected to that power of the mask that surpasses time itself as he witnessed and that Rimuru is, therefore, the key to the "truth of the world."

He tried to get summoned when Rimuru performed a Greater Demon Summoning within Ramiris' Labyrinth. He failed because Beretta, his own descendant, answered the summon first much to his chagrin.

Finally, when Rimuru did another Greater Demon Summoning to catch the hiding Razen, Noir managed to answer the call alongside two of his Greater Demon followers. After some further events, Noir finally received the name "Diablo" from Rimuru and evolved into a Demon Peer.

Trivia

 * Rimuru named Diablo after the Supercar.
 * Diablo's naming took half of Rimuru's total magicules reserves even though his total magicule capacity increased by ten times after becoming a demon lord.
 * It's implied by Beretta and Guy Crimson that black colored Demons are the smallest in number as Diablo doesn't like to create "descendants."
 * Diablo's happiest thing to do is butler/housekeeping work for Rimuru. To him, this is way more important than tasks like toppling nations and political machinations.
 * Diablo is a loner who doesn't like having subordinates unless it is for the sake of Rimuru, or to pawn off tasks that require him to be away from his master.
 * Diablo is Spanish for "Devil."
 * The only thing that scares him is the thought of losing his job as Rimuru's secretary. From time to time, he remembers when Rimuru said he could leave (only shortly after being summoned), and this sends shivers down his spine.