Honestly this wasn't one of the tropes I'd hoped to see pulled out of the bag on this one.
This would be much more compelling if the heroine was put at odds against our protagonist by some subtle twist of fate, now or later, or heck, even if they even just got along instead. Or the heroine being another quiet reincarnee with her own problems and goals, that could have worked too.
But "a wild annoying heroine appears"? It just clashes kind of brutally with the theme so far (which has been fairly sombre, no?)
Then again, I don't know where the author is going with this. It could still be cool.