The characterization of Diavolo in season 3 of the og game was peak to me. I know his cards are pride, and thats definitely there to see, but he's soo envy-coded to me.
Throughout almost every interactation we have with him, it's about him wanting others positions. He wants the brothers as friends, he wants Lucifer especially, and he inserts himself into their lives to get a bit of their love for himself. When MC becomes a part of their family, Diavolo is jealous of both sides. He wants to be accepted as a part of their family like MC, and he wants MC to himself like the brothers seem to claim them. He wants, he wants, he wants.
Thats not to say Diavolo isn't a genuinely kind person! He goes out of his way to listen and help others, and he's a respected ruler for a reason. But a core aspect of his character is his desire to be someone else, he even makes a point to tell Belphie he's "a child who never gets to experience anything." All those stupid events and parties he railroads others into is his attempt to mimic what others are allowed to have. Being a prince isn't enough, he's always going to he envious of what others have that he doesn't.
Diavolo's envy is also what makes him a fantastic counterpoint to Levi. They BOTH spend their time wishing to be someone else, the difference is that while Levi gains more confidence alongside MC, Diavolo gets worse.
Diavolo, by the point of the og game, has accepted that he won't be a part of the brothers family. But then, MC goes and does just that. MC acomplishes what he's wanted and been envious of for millenia in just a year. He wants what their family has, and he's also envious of the brothers having MC to themselves. It's just an absolutely tragic dynamic, seeing as Diavolo is just unable to understand why they don't accept him. Diavolo just doesn't have the social graces to understand how MC was able to help the brothers, and how they had to suffer for it. All he's able to see is that the family he's yearned for, and the human he loves are intertwined in a way he doesn't know how to replicate.