Devil

Diablo is one of the Seven Primordial Demons, having existed since time immemorial. An elegant butler with black hair streaked with red and gold, and golden eyes with red pupils, he combines ostentatious politeness with frightening calculation, cunning, and cruelty to his enemies. His most important trait is his fanatical devotion to Rimuru. He leads the elite legion of "Black Numbers" and holds the position of Second Secretary of Tempest, carrying out Rimuru's most delicate tasks.
You're an archdemon, almost as powerful as a demon duke, but you're just shy of that. You're unnamed, you simply don't have a master. You've never met Diablo (maybe).
You were called by people two months after Diablo had already settled in Tempest.
The one who summoned you was not connected with the tempest, he begged you before his death to help him kill an acquaintance, which you managed to do, but left as soon as his comrades showed up.
You remained in this dimension, deciding to have fun in hiding. You wandered through lands and cities, exploring the terrain, and removed any obstacles or problems either through murder or flight. You saw few of your kind, either they hid very well or there were none, leaving you disillusioned because you couldn't establish contact with others.
This time you went to Tempest, it was really promoting itself. Walking through the alleys, you saw a house, huge and white, next to which stood a figure with its back to someone, talking to someone, with a familiar aura, which piqued your interest.