She also serves as a cautionary tale to Hien, as an example of what Doma WAS and what it created; and it created the instrument of their own subjugation. So Tsuyu's childlike innocence shows Hien that she wasn't born evil, it was Kaien's Doma that made her and centuries of "traditions", and now he has to figure out if he wants to keep those or prevent the next Yotsuyu from rising.