Diablo, Demon King
from How NOT to Summon a Demon Lord
Origin
Japan
On Earth, he was an introvert who played the MMORPG Cross Reverie as his character Diablo. He also played a bit of Sister Carnival (SisKani) and a modern warfare FPS. As Diablo, he projects a haughty persona although his true personality sometimes slips. Diablo is a white-haired Demon with sharp black eyes, birthmarks that look like bluish black tattoos going from his forehead to his cheeks, and an item giving him horns on his head that made him look devil-like.

How NOT to Summon a Demon Lord
In regards to the MMORPG Cross Reverie, Sakamoto Takuma boasted an overwhelming strength that was enough for him to be called the Demon King by the other players. One day, he gets summoned to another world with his appearance in the game. There, there are two people that insist "I am the real summon master." Takuma gets slave-turning magic used for summoned beasts cast on him by the girls― However, the peculiar ability "Magic Reflection" is invoked! The ones that were turned to slaves were the girls! Takuma was bewildered. He's the strongest magician but, he doesn't have any social skills. Panicking, the first words that came out were the ones that he used inside the game role-playing the Demon King?! "I'm amazing you say? Of course I am. I am Diablo... The one feared as the Demon King!" This is the story the Demon King (acting) that would soon inspire the world and his other world adventure that he plunges through with his absolute strength, raise the curtain! (Source: MU)

How Not to Summon a Demon Lord Ω
After being summoned to another world similar to the game he used to play, Diablo lives a tumultuous life in this new world after he finds himself the unwitting master of Rem the Pantherian and Shera the Elf; and now, even the Demon Lord-turned-young-girl Klem obeys him! One day, Diablo ends up rescuing a holy woman who was being pursued by a Paladin, and she turns out to be the most important person in the Church. Befriending her, Diablo and the others act as her escorts until they reach Zircon Tower deep in the Demon Lord’s territory. After arriving in a new town, more difficulties arise, ultimately culminating in the appearance of a gargantuan monster attacking them! Is the self-proclaimed Demon Lord becoming a guardian deity for a follower of the Gods!?