Diablo

Diablo is the Primordial Black, one of the seven oldest and most powerful demons in existence, and Rimuru Tempest's most devoted servant. Originally known as Noir, he has existed since the dawn of time alongside the other Primordial Demons. He became fascinated with Rimuru after witnessing his summoning and willingly entered his service, an act that shocked the demon world since Primordials are considered near-godlike beings. Diablo is refined, sadistic, supremely loyal to Rimuru, and possesses intelligence and power that rival the strongest Demon Lords. He serves as Rimuru's Secretary and one of his top three subordinates.