Imagine that Mc is the only person capable of saying no to Diavolo normally, without any fear or consideration for his status. For example, imagine that when they were in the human world Diavolo proposes something grandiloquent and attention-grabbing, but Mc tells him firmly “No, Dia, we're not going to do that”. Or in the creation of a new event, the prince proposes to alter all the schedules to do something totally new and unknown, everyone is racking their brains to tell him it's not a good idea and the human just comes up to him and says “It's not a good idea, let's think of something else”.
And while all the demons around look at Mc with fear, reproach or disbelief. We have the future king of Devildom looking with heartfelt eyes at the human. Because in Devildom he can do anything, no one can deny him anything reasonable or not, most demons are unable to even show disagreement against him. But Mc is not afraid to speak their mind, they are not afraid to tell him how bad some ideas are, how dangerous or senseless they are, they are not afraid to go against him. And that just makes him, for once, feel like someone treats him as an equal, sees him as an equal. And that just makes the demon appreciate the human more and more.
Noble Demon: What do you see in that human, my lord?
Diavolo: *heart-shaped eyes* They tell me no.
Mc: *saluting him with a smile*
Diavolo: *sighing* They are perfect.