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 a Demon God and a loyal servant of Rimuru Tempest. After being named, he has evolved from an Arch Demon into a Demon Duke; and after being awakened into a True Demon Lord (and Rimuru turning into a Great Demon Lord), he evolved into a Demon God.

As one of the most powerful figures in Tempest, he is one of Rimuru's Twelve Patrons and also the current leader and founder of the Black Corps.

Among Rimuru's subordinates, he is one of the strongest three individuals, alongside Benimaru and Zegion.

Appearance
As an Arch Demon, he was a beauty whose gender was indeterminable. He possessed crimson hair 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.

Personality
As a summoned demon who has been given a name by Rimuru, he is extremely loyal to his master. When two of his subordinates were sacrificed to fuel Rimuru's revival spell, he felt envious that they could be of use to his master. His aim is to be Rimuru's, right-hand man.

Aside from treating his master humbly, he often speaks politely to both his allies as well as enemies.

Past
Diablo has always been an absurdly strong demon who loved to fight. Since the beginning, he would battle Angels at every meeting in a long unavoidable fight. These fights were too destructive, so they would only fight every 500 years for seven days. Diablo would get very bored during this intermediate times. And since he was a demon, (angels as well) made of pure energy, unless he had a physical body he could not interact with the physical world.

For him, fighting simply meant trampling his opponent. Even in fighting the Angels he would massacre them.

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." - He reveals so while battling a Templar Knight.

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 a the names of all Demons named by Rimuru except Beretta, comes from a car, in this case, the Lamborghini Diablo.