Demonic OCs need a core that isn't just 'evil,' you know? The ones that stick with me have some internal logic that makes sense, even if it's terrifying. Like, maybe they're not trying to destroy the world because they're a villain, but because they genuinely believe existence is suffering and ending it is a mercy. That kind of tragic, philosophical underpinning creates way more interesting conflicts than just wanting power.
What I look for is a motivation that feels almost human, just twisted. A demon obsessed with collecting beautiful moments of pure despair because it finds them aesthetically pleasing, not out of malice, but like an artist. It's the difference between a cartoonish bad guy and a force of nature with a disturbing point of view. The best ones make you catch yourself almost agreeing with their warped perspective before you remember they're a monster.
That internal consistency is everything. If they're a liar, show why they believe truth is weak. If they're cruel, let it stem from a perverted sense of order or a deep, ancient wound. Give them a code, however horrifying, and stick to it. That's when they become unforgettable, because you can predict their actions based on their broken logic, not just the plot's needs.