Oc was cursed when before she was born, she won't live past 25. She'll eventually go numb and lose all of her senses but first to go was she was born without sound.
Or oc was cursed like Inumaki from JJk. She has to be careful with her words, they can become deadly.
Oc is the guardian of Muichiro, she took him in when he arrived and helped him recover. She is the only one he can remember her name. She becomes his mother figure. This surprised everyone because oc is the most withdrawn hashira out of everyone. She barely smiles at anyone and only every really shows herself when called to the hashira meetings.
Oc was Sanemi's tsuguko and became a hashira. She is the same age as Muichiro. She was saved by Sanemi when her whole village was slaughtered by a demon and she was the sole survivor. Maybe she is blind and can feel her the nature/aura around her.
Sorry if this is becoming annoying...