She's the one twin that he relied the most. Later chapters shows her eagerness to always be relied on by him and always disappointed if he just barely praised her(like, "thanks, now go back to work").
In the chapter before the last, we finally get her POV(we never get the other twin POV) as she is writing some kind of memo. When her twin comes to tell her to stop writing and get something done, she is reluctant as she never approved it and says tomorrow is "the worst day of her life"(her twin says "the best day of her life" at the same time).
The last chapter is the wedding ceremony of her parents(MC never actually legally maried his wife), and she is the only one who isn't very happy about it.