Responses
That makes sense, Mephisto and Sully could go beyond the nether while other demons are so shocked about humans existence.
Maybe Sully is the only demon who could cross to the human realm and since his mana was called ''Delkira's mana'' maybe he's a whole different entity.
Maybe that's why the ex-thirteen crown music guy said that Sully could never be the demon king, because he also exists out of the norms of the demon world.
And maybe that's why he is "cursed" as the other guy said, because he probably can't interfere in a important way, even when people that he loves need him. Maybe that's why he needs opera to act as his hands.
Or he is "cursed" because he is supposed to act one way or to have one responsibility that it's his reason to live (like mephisto raise kings as if it was his reason to remain alive) and becoming the demon king is not within that reason of his.