one of the easiest ways to tell what a superhero's whole deal is (their thesis) is to look at their nemesis (their antithesis). the fun thing about a nemesis is that they are the exception to the rule, the outsider to the hero's worldview.
Superman is a refugee raised in the midwest who was told that people are inherently good and that they just need help sometimes. To counter this you give him an ultra-capitalist villain who could save the world, but won't unless he gets all the credit for it, what's the point of saving the world if you can't put the corporate logo on it. One of the biggest misunderstandings when writing Lex Luthor, in my opinion, is putting him in his iron man armors. They constantly have him put on a suit and try and beat Superman and fight him. Which he will never win- you don't beat Superman in a fight. No, he is actually at his most threatening when he isn't in the armor because then, Superman can't touch him. What is he gonna fight Lex? Punch him across the room? So much of Superman's rogues gallery is about might and making someone who is "totally gonna be an actual threat this time we promise". So having Lex be someone who sidesteps that and refuses to fight Superman on those terms is when he is at his best. Worse that happens? Superman kills him and he still wins. You look at how the hero wants to fight, the worldview they want to have, and then place someone in opposition to those things.
Batman is much the same. He wants to defeat crime and save people, he wants to believe everyone can get better. So you make Joker, someone who is horrific and loves being that way, he won't change because he doesn't want to- he loves being who he is. Almost every other member of the rogues gallery has some tragedy that keeps them going, something unfair that if corrected would mean they could stop, they could heal. The Joker is incapable of that so you get this endless cycle that works great for this style of story. But most importantly who both these characters are will force them into conflict.
there are lots of characters lacking in a proper nemesis because they don't create a sharp enough contrast to work. this kind of diametric reaction where when one acts, it is also defining the other. i think this is a good exercise for character writing and creating antagonists is to figure out your hero first; what they want, how they fight for that, what they believe in. Then subvert, convert, and pervert those ideas until you have proper opposition.
