Superman. I like him, perhaps because I come from an era when comic books were safe for kids. But there is one interesting think about Superman. As he was originally conceived, it was very hard to write interesting stories about him.
Superman was basically invulnerable, with no weaknesses. We felt he was quite heroic, but everything was so easy for him, was he really heroic after all? He could pick up street criminals by the bunch and drop them at the jail. Of course he probably had to go back to pick up eyewitnesses so the criminals could get convicted. But it wasn’t really a challenge for him.
The writers of Superman added kryptonite so that Superman had a point of vulnerability. It’s kind of silly that rocks from Superman’s destroyed home planet would be harmful to him. And that so many villains could obtain kryptonite. But by making Superman vulnerable— at least to one thing— it made it easier to identify him. And he could be really heroic by taking real risks in order to save someone.
The other thing that the writers of Superman had to invent were supervillians. Ordinary, realistic street criminals were too easy for Superman to defeat. So there had to be villains with superpowers of their own, or who were evil geniuses who could figure out how to seriously endanger Superman or thwart his efforts.
The rule we should learn from this is a real hero needs to be vulnerable in some way, and needs to have an opponent who can actually harm or defeat him.
This can be a problem in some fantasy fiction. Writers might create heroes with amazing magical powers so that the reader wonders: why doesn’t he just use his magic? Unlimited powers in a hero lead to boring stories where the hero is unchallenged. Or unrealistic stories where the hero doesn’t attempt to use his magic powers to accomplish his goals without any real reason to not use this power.
A villain that seems too powerful to defeat does work in fiction, as long as the author plants clues that the villain has a weakness, a limit to his powers, or a way to be defeated. The Harry Potter series works in part because we know Lord Voldemort had limits. He couldn’t just sit in his lair with magic wand out, chanting ‘Aveda Kadavera’ and have his distant enemies all drop dead. We know from the beginning of Harry Potter that Lord Voldemort was defeated once, and that he had opposition as well as supporters.
Writers that don’t think their story through, give both villain and hero non-unlimited sets of abilities, and arrange defeats-of-villains that make actual sense in the story world don’t manage to create entertaining stories.