Diablo

Diablo (ディアブロ diaburo), also known as Noir ( nowaru, lit. "Primordial Black") is one of the seven Demon Primordials 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 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.

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 or 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 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 and Beretta, of his own line of Demons, 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.

Demon Lord Awakening
By offering the soul-less corpses of the remaining 10,000 Farmus soldiers, Rimuru successfully summoned Noir and his two Greater Demon 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 Demons did their work promptly, successfully capturing Razen alive.

When the secret revival art was lacking the necessary energy, 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 Demon 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 Demon 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 Demon Peer.

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

The Octagram Soars Brightly
Diablo was then given the task of investigating Farmus and cripple its economy and overthrow the current Farmas king.