Interesting new arc, it could go several ways:
-Actual ghost.
-Maria's mind playing games with her.
-She maybe also reincarnated, like if allbrith did maybe she too.
-The most likely it is her little sister aka the reason of why she had to kill allbrith, maybe she is there in search of why her sister died, or maybe she is working with the evil noble without knowing he is the reason of why her sister die, or maybe she is there for revenge.