Ever tried watching a horror movie where the monster is just pure evil? It's boring after fifteen minutes. Same with a demon OC. The darkness needs texture, not just pitch-black paint. I built mine around a contradiction: a hunger for human warmth she's physically repelled by. She'll meticulously arrange a victim's belongings after a kill, creating a perfect tableau, because she's obsessed with the domestic tranquility she can't have. That little ritual makes the violence feel more unsettling, right? It's not random brutality; it's a screwed-up form of yearning.
Where folks mess up is making the relatable trait a 'good' one. It doesn't have to be. Maybe your demon is incredibly loyal to its infernal patron, or follows a twisted code of honor, or finds deep aesthetic pleasure in a specific type of decay. Readers connect to commitment, to passion, even if the object is vile. The darkness is the 'what,' the relatable bit is the 'why' and 'how.' Just don't give them a tragic backstory that completely excuses everything—that's a cop-out. Let the tension live. My demon's relatable trait is her fastidiousness. Her evil layer? She uses the skin of her victims as parchment for her memoirs.