Why is nobody actually talking about the topic at hand? Know what? Whatever. @Mikasa_1412 gave some lovely insight to how to build a believable villain, however with everything they said in mind, I’d like to add that on some occasions, depending on the story, it’s also good to have a villain character that’s just evil because they’re evil. No explanation required. It’s not as interesting but sometimes it’s just what the story requires.
As for protagonists, I cannot stress enough the fact
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that they NEED. FLAWS. Like true, good to honest flaws that will provide obstacles in their journeys. Not just things like, say, they’re clumsy. That’s good if it’s not the only thing, but clumsy alone doesn’t do much for making them a compelling character. The flaws can’t just be physical, by the way. Physical flaws are fine (and encouraged because I’m tired of physically perfect characters) but the internal flaws are what typically really make a difference in the story itself.