Diablo

"Your name shall be Diablo. Receive that name, and become my guardian deity!"

- Rimuru naming the Arch Demon upon Rimuru's awakening as a True Demon Lord

"Demon Lord" Diablo 「 ディアブロ,  Diaburo」 is the only subordinate to be named by Rimuru Tempest on his seventh occasion of naming; meaning, he was not part of a group when he was named. Originally an Arch Demon who was summoned during Rimuru's ascension into a Demon Lord, he was absolutely enthralled by Rimuru's vast powers and wished to serve him even after the task at hand was completed. He worships Rimuru like a God and constantly competes against Shion to be his closest aide and number one subordinate. He is also possibly the only subordinate to receive ability upgrades of the same attribute as Rimuru (Nihility) directly from him.

He is also the founding leader of the Black Numbers, Rimuru's own personal corps, which is the most powerful corps in Tempest, even if it's the smallest. Later on, he received the Title of a "Lord" as one of Rimuru's Twelve Patrons and holds the position as one of his three strongest subordinates, alongside Benimaru and Zegion.

Arch Demon
As a spiritual existence, unnamed Diablo had no physical form. But whenever he was summoned and took on flesh, he was a beauty whose gender was indeterminable. He possessed crimson hair (WN) while his eyes had golden irises and black scleras. Often appears to be elegantly dressed in a professional butler uniform and carries himself with an air of dignity.

Demon Noble & Demon God
Diablo's change in appearance is possibly the least drastic after any evolution. But it is also possible that he did undergo change and the author never explained. Regardless, the appearance change is likely to be negligible. The only to note would be that Diablo covers himself in a black cocoon of magic every time he undergoes evolution.

Personality
Diablo is a natural loner, or rather, a lone wolf. Ever since his Arch Demon days, he was not one to have comrades nor subordinates. However, after becoming part of Tempest, he showed signs of cooperative operation, although that is most likely because he wishes for the absolute best outcome for his master Rimuru.

As a Demon, Diablo can't help but want to fight and crave for power. It is also because of this that he always killed all people involved in summoning him; both the target of the summoners and the summoners themselves. Thus, being summoned by Rimuru was the best thing that ever happened to him since he got both power and also strong opponents he could use them on. It is also because of Rimuru that he was able to see a 'sky beyond a sky' and realized there was always someone better than him. Ever since then, he has been aiming to be Rimuru's only right-hand man, attempting to decipher every intention and the thought process of Rimuru, as an attempt to understand his master and become his closest subordinate.

He is one of the few subordinates who worship Rimuru like a God and always wishes to be useful to him. This is why he was really envious of the two Greater Demons who were summoned alongside him, to be used as sacrificial fuel for Rimuru's revival spell, since they were useful to him in a critical moment.

Diablo is also some who values elegance and beauty in everything, even in fighting. When fighting against the Knights Templar, he would complement them if a technique was graceful. This is likely another reason why he finds it unpleasant whenever he finds Shion and Shuna arguing over who gets to wake Rimuru up in the morning. Another effect of having this attitude is that he tends to speak politely to both his allies and enemies, despite his immense power and pride.

Background
Diablo is one of the oldest Demons in existence, alongside Guy Crimson and Testarossa. Meaning, that he has always been an absurdly strong demon who loved to fight for a very long time. After it became a rule that the unavoidable fights of Angels and Demons were to only happen for 7 days after an interval of 500 years, Diablo would always be bored living in the underworld. Regardless of whether it were the Demons in the underworld or the occasional summons, he got from the physical world or even the battles against the Angels, fighting would always be one-sided trampling.

Because of there being no existence higher than him, Diablo had basically stopped chasing after power and simply wished for a good challenge. But when Rimuru had used the High Demon Summoning, he could just feel the immense power from the portal. It was then he realized that there were existences beyond him. And when he thought that this being was about to rise as a Demon Lord, even he couldn't see the end of this person's power. As the energy he had used up to create a body for himself was replenished using Rimuru's magic, that body completely adapted to him perfectly. And finally, the decisive factor was when he was named. Thus, he wished for power once again. Power to be the number one subordinate. That was the symbol of the most useful and trustworthy. That became his new aim.

Birth of a Demon Lord Arc
By offering the corpses of the Blood Shadows and the 15,000 human soldiers, Rimuru successfully summoned an Arch Demon and his two Greater Demon subordinates. Using the corpses, they were able to successfully materialize and his first order for them was to capture the three escaping Blood Shadows and bring them to Ranga.

When the secret revival art was lacking the necessary energy, the Arch Demon offered to sacrifice his subordinates, stating that they were delighted to be of use to Rimuru. Hence, when Rimuru awakened, he was grateful for his efforts and allowed the Arch Demon to become one of his comrades. When Rimuru found out the Arch Demon didn't have a name, he named him "Diablo," tasking him to become his guardian deity. After Diablo was named, a black cocoon enveloped his body, creating a perfect evolution posture.

It is later revealed that he had created this body by using a massive quantity of magical power that he had saved for a long time and thus transformed into a Demon Duke. This magical power was immediately replenished through his link with Rimuru. He believes that in the Demon world there is no existence higher than him. But even he cannot fathom the depths of the strength of his beloved Master Rimuru.

Saint Demon Confrontation Arc
"I am not an Arch demon like you had previously said, but a Demon Duke."

- Diablo revealing so while battling a Templar Knights It is later revealed that he was no longer an Archdemon, but a Demon Duke. According to the existing records of Demonology, a Demon Duke is a legendary existence and is supposedly ranked higher than a self-proclaimed Demon Lord. There has been no other Demon Duke summoning previously documented.

Trivia

 * He has red hair color in the Web Novel and black in the Light Novel.
 * His name, as well as the names of all Demons named by Rimuru except Beretta, comes from a car, in this case, the.
 * Diablo is also 'One hell of a Butler'.
 * It is implied in Episode 23 of the anime that Diablo was supposed to be answer Rimuru's summoning in the labyrinth instead of Beretta, but he had gotten ahead of him.

Gallery
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