Responses
It's probably related to her personality. She cared so much about someone else like how she conceals her sickly palor so people will smile or protect Keigetsu dignity at all cost but barely thinking about her own consequence. Her will to live also pretty poor, in previous arc Keigetsu feels fine in Reirin body even after crossing the river, but Reirin almost broke down just from the thought of losing someone, if not for the robust body she might fainted already. Since you know there's saying how the mind also affected the body.
Kou reirin said that she was feeling more healthy due to keigetsus fire while her body is of earth affinity.
So maybe the longer keigetsu is in her body, the more likely on the track for her ailments being treated.
I don’t think she was just born frail, I’m thinking something happened or is happening that’s affecting her body and slowly deteriorating.
It might not make sense since shes a pro at herbs and whatnot, but I refuse to believe they’re gonna kill her off when she’s older.
That would end meeeee.