Alright, so building an OC for Demon Slayer isn't just about giving them a cool sword style—though that helps. The world’s so rich with its Taisho-era Japan setting and the whole demon-slayer system that you gotta anchor your character in that reality. I always start with the breath style. Think about what environment or philosophy could spawn a new one. Maybe someone who grew up near hot springs developed a 'Steam Breathing' technique, using mist and heat. That automatically creates a backstory: why there, and who taught them?
Then, the tragedy. It can’t just be a dead family—everyone’s got that. Make it specific. What if their family were traveling performers, slaughtered on the road, and the only thing they saved was a single, bloodstained prop mask? That gives them a unique drive and a physical memento. Tie their motivation to a lesser-explored part of the world, like maybe they suspect a demon is impersonating a human in a specific trade, like a paper-maker or a train conductor. The Corps sends them undercover, which creates instant plot hooks and forces them to use their wits, not just their blade.
The real trick is weaving them into existing lore without breaking it. Have them fail their Final Selection the first time, or get their Nichirin sword from an unconventional blacksmith who uses an old, frowned-upon method. Those small details make them feel like they’ve always been there, scraping by in the margins of the story we already know. Mine ended up with a sword that turns cloudy grey under moonlight, which annoys the hell out of the regular swordsmiths.