The one that followed her to the North after getting married. I found the way she talked here odd
http://www.mangago.zone/read-manga/the_duchess_with_an_empty_soul/nbt/iur_chapter-26/pg-61/
Hmm..... from my experience reading many mangas, I suspect either:
One of the prince who have yet to appear and the reason is to snatch the crown from King&CrownPrince and rule over the world.
OR
Young ruler of other kingdom with the same purpose as the reason above...
LOL, or we can search all chapters and look for someone with yellow/golden eyes who kinda seems suspicious.
This is a pretty great story so far!
Thinking who the main villain might be? It seems like someone who doesn't know she has summoning powers. When "that" person was talking with the auctioneer, he/she only mentioned the knights as being the ones to protect the MC and not that she can fend for herself as well.
The guy with the red hair and the princess whom I have suspected as being the main villain know about her abilities. Also, the lady with the red hair doesn't seem like she'll be against her anymore seeing as how she was struggling with where society put her too, so it's not her either.
I'm now suspecting either a new character or the maid.