There was no extra "antagonist" in the WN (from Bertia's POV). Her enemy was Heronia alone. The one who dirtied Bertia's name was also Heronia herself.
In this manga, a new character was introduced (the one Cecil is taking revenge on in this chapter).
Of course I had always wondered who the hell the girl that kept on having the whole glaring scenes was the whole time (since I've read the WN, and she didn't exist there), now I think I see how it is. I guess the manga is planning on changing Heronia's ending by making her cause less wrongs, at least when compared to what she had done in the WN.
To be fair, her ending was a bit sad there (although it didn't really matter much cause it was all about Cecil x Bertia, but still).