I've seen so many Demon Slayer OCs where the tragic backstory feels like it's ticking boxes. Parents killed by a demon, check. Vengeance motive, check. Honestly, the most memorable ones I've read aren't about the scale of the tragedy but about the weird, specific human details that get lost in it. Like an OC who's actually terrible at swordplay but joins the Corps because he's the only one left who remembers his little sister's favorite lullaby, and he's convinced if he kills enough demons he'll forget the tune and finally let go. The grief isn't a weapon; it's this heavy, awkward thing he keeps tripping over. His breathing style could be something clumsy and defensive, born from panic, not talent. That feels more real to me than another prodigy with a dead family.
You could also play with the idea of a former Kakushi who got promoted after his entire squad was wiped out. He's not a natural fighter, but he knows logistics, terrain, and demon behavior inside out. His strength isn't in flashy techniques but in preparation and exploiting small advantages. Maybe he's resentful of the 'real' slayers with their flashy breaths, or maybe he's terrified they'll all die because he missed a detail. That kind of internal conflict writes itself.
Ending on a massive, world-shaking tragedy is easy. Making us care about a character who lost something small and stupidly precious is harder. Focus on what the character lost, not just that they lost.