Even if Rosaria is really the Saintess' daughter, we don't know if the Duke is her father. After all, she would have to be Cordelia's twin sister and as far as we know, Cordelia is Orixa's daughter.
Even if Rosaria is really the Saintess' daughter, we don't know if the Duke is her father. After all, she would have to be Cordelia's twin sister and as far as we know, Cordelia is Orixa's daughter.