Diablo/Relationships

Rimuru Tempest
Diablo is fanatically loyal to Rimuru. Worshiping him like a god.

Shion
They are rivals for being the "primary" secretary for Rimuru, though aside from that they both share a similar fanaticism for Rimuru, though for different reasons.

Benimaru

Diablo and Benimaru seem to be on friendly terms with each other.

Souei

Diablo and Souei seem to be on friendly terms with each other.

Hakurou

Diablo and Hakurou seem to be on friendly terms with each other.

Goblin/Hobgoblins
Rigurd

Rigur

Gobta

Humans
Shizue Izawa

Diablo respected Shizue Izawa as a Wreathy Hero and opponent when they battled each other. He even saved her life from the demons he was contacted to hunt down and kill.

Black Numbers
Diablo's faction of subordinates, numbering several hundred. As a show of loyalty to Rimuru, Diablo returned to the underworld to recruit only the best of the demons to serve Rimuru. This included the Violet, White, and Yellow progenitors. They were named allowed with at least one lieutenant each. From this group, Diablo has Venom serving as his lieutenant.