Honestly, the worldbuilding in 'Demon Slayer' gives you so much to play with right off the bat. Don't overcomplicate the setup. Grab a few friends who love the series, pick a Discord server or a Google Doc, and just drop your OCs into a simple scenario. Something like 'our squad gets a mission to a village with strange disappearances' works perfectly. You don't need fancy rules—just agree on the vibe and start writing what your character does and says. It's more about capturing the atmosphere of desperation and the clash between human fragility and demonic power than sticking to a rigid system.
We found that assigning everyone a Breathing Style early helped define roles. One of us went with a Water Breathing user who was overly cautious, another made a boisterous Flame Breathing hothead, and I played a support-type using a modified Breath of Sound for reconnaissance. The friction between their approaches created instant story. The key is to embrace the melodrama and intensity of the source material; let your characters get hurt, have emotional breakdowns mid-fight, and deliver those over-the-top, determined speeches. It's cheesy in the best possible way.
Also, don't shy away from using established characters like the Hashira or Muzan as distant figures or narrative catalysts, but keep the focus on your original slayers and their personal struggles. That's where the real heart is. We ended up with a whole subplot about my character's guilt over a lost sibling that mirrored Tanjiro's drive but twisted it into something more self-destructive. Just start typing and see where the demon blood takes you.