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 Daemons and an Executive of the Jura Tempest Federation, serving as a secretary and Butler to Rimuru Tempest. He is the leader of the Black Numbers and is Rimuru's most powerful subordinate among the Twelve Guardian Lords.

Arch Daemon
Black hair with streaks of red and gold, as well as golden eyes with black sclerae, 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.

Daemon Lord
Diablo's change in appearance is possibly the least drastic after any evolution, with only his princely garments being replaced by a Butler outfit, reflecting his wish to serve Rimuru. Despite that, however, his dignity did not decrease, but rather increased instead.

Personality
Diablo is whimsical but also very calculative when he wants to be. He usually displays an absolutely composed attitude, but whenever anyone insults Rimuru, his polite exterior turns into one of cold malice as he proceeds to punish the "sinner" in the most excruciating way imaginable, unless it goes against Rimuru's orders. However, when one has a good relationship with Rimuru, Diablo usually treats them in an amiable way, though that does not mean he won't manipulate them for his own ends, "his own ends" usually referring to being close to Rimuru more often.

Diablo follows Rimuru's orders down to the letter. When tasked with increasing the difficulty of Framea's test as a guard for guests to Tempest, Diablo had summoned a moderately strong monster. Upon seeing Framea's will to fight despite being paralyzed by the creature's goo, Diablo sent the creature away and returned to report back on her actions. Should there be any "grey areas" 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 daemons, 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 attempted a desperate summon to save Shizu's Students from a Phantom, only for the unnamed Diablo to offer his aid for what was practically 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 his whims and very rarely showed 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 Daemon despite having enough power to evolve into a Daemon Lord already.

One day when he personally answered a Demon Summon of the Silver Wings 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 had an incomprehensible nature. 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 Daemon Summoning within The Dungeon. He failed because Beretta, his own descendant, answered the summon first much to his chagrin.

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

Trivia

 * Rimuru named Diablo after the Supercar.
 * Diablo is the Spanish word for "Devil".