He is one of the 7 primordial demons, the black progenitor of black lineage demons, one of the pinnacle power of the underworld, he was name once as noir but got named "diablo" by rimuru tempest after being his subordinates and his loyal butler, diablo devotes his entire life to rimuru as he's loyal for him, he would do anything for him

Diablo is whimsical but also very calculative when he wants to be. He usually displays an absolutely composed attitude, but whenever anyone insults Rimuru