Diablo

Diablo (also known as Noir or the Primordial Black) is one of the most powerful and memorable characters in the anime That Time I Got Reincarnated as a Slime. He is originally one of the seven ancient Primordial Demons — specifically the "Primordial Black" — incredibly ancient, immensely strong entities that existed long before most of the story's events.

In the anime, Diablo is summoned by Rimuru Tempest during a critical moment (around the events involving the war with Falmuth in Season 1's later episodes and carried into Season 2). Rimuru intended to summon a powerful greater demon for assistance, but Diablo — drawn by Rimuru's immense potential and character — forcefully answers the call himself. Upon being named "Diablo" by Rimuru, he evolves dramatically (from an Arch Demon to a Daemon Lord/Demon Peer level), pledging absolute, fanatical loyalty to Rimuru as his master.

Diablo serves as Rimuru's personal butler and second secretary in the Jura Tempest Federation. He is calm, highly polite, and impeccably mannered in his speech and behavior, often with a refined, almost aristocratic demeanor. However, beneath that elegant exterior lies a ruthless, calculating, and sadistic side when dealing with enemies or threats to Rimuru — he has no hesitation about torture, manipulation, or elimination if it serves his master's interests.

He is widely regarded as Rimuru's strongest subordinate (or at least one of the absolute top tier among them), with overwhelming combat power, vast magical knowledge, strategic intellect, and unwavering devotion. His loyalty stems not just from being named, but from genuine admiration for Rimuru's kindness, potential, and vision — something that reignited his interest in the world after eons of boredom.

In the anime adaptation (up through the available seasons), Diablo is voiced with a smooth, charismatic tone that perfectly captures his mix of sophistication and underlying menace.