I'm sure there will be an arc just for Arjen. And the story is slowly breaking apart the thought that twins have to be the same all around, (same thoughts, same likes, same dislikes). Not every twin is like that.
Plus, it's stated that Arjen has more imperial blood and Arien has more of her mother's blood. If the author was able to make the twins reconcile with their father, then I'm sure the author will be able to show the two twins distinctive sides.
MY QUEEN MY QUEEN I ONLY HAVE EYES FOR YOU FUCK THE BOYS I LOVE YOU ANSNSNSMSNS BEAT ME UP WITH THAT BATE PLEASE