Diablo

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

Arch Daemon
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.

Daemon Lord
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 amiable, 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 Flamaire's test as a guard for guests to Tempest, Diablo summoned a moderately strong monster. The moment Flamaire 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 Flamaire'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 progenitors) to serve his master; the creation of the Black Numbers, who became the de facto strongest fighting force in Tempest.

Backstory
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 Daemon despite having enough power to evolve into a Daemon Lord already.

One day when he personally answered a Daemon 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 had the 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 of infinity, when the mask came into Riumuru's hands, Noir came to the conclusion that Rimuru is connected to that power of infinity he witnessed and that Rimuru is, therefore, the key to the "truth of the world."

He tried to get summoned by Rimuru when Rimuru made a Daemon Summon within Ramiris Labyrinth, but he failed and Beretta of his own line of Daemons, answered the Summon first, to Noir's chagrin.

Finally, when Rimuru did another Daemon Summon 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.

Demon Lord Awakening Arc
By offering the soul-less corpses of the 20000 Farmus soldiers, Rimuru successfully summoned Noir and his two Greater Daemon subordinates. Using the corpses, they were able to successfully materialize and Rimuru's first order for them was to capture the hiding Razen and bring him to Ranga. Mesmerized by Rimuru's Soul the three Daemons did their work promptly, successfully capturing Razen alive.

When the secret revival art was lacking the necessary energy, the Noir offered to sacrifice his subordinates, stating that they were delighted to be of use to Rimuru. Hence, when Rimuru awakened and found out that the Arch Daemon he summoned did his job properly he was grateful for his efforts and allowed Noir to become one of his vassals. When Rimuru found out that the Arch Daemon didn't have a name, he named him "Diablo." After Diablo was named, a black cocoon enveloped his body and the now named Diablo evolved into a Daemon Lord.

Afterwards, Diablo was allowed to attend the meeting to discuss their future plans.

Trivia

 * Rimuru named Diablo after the Supercar.
 * Diablo is Rimuru's strongest named subordinate.