The Empyrean series seems to be building to a redemption arc for Dain Aetos, but I wish Fourth Wing did a better job of setting this up. Dain is introduced early in Fourth Wing, and Rebecca Yarros establishes that he and Violet have been close friends for years. Dain joins the Riders Quadrant earlier than Violet, and the experience changes him and not necessarily for the better. Violet spends much of the first Empyrean book grappling with the new dynamics of their relationship. As Fourth Wing continues, their bond deteriorates until he betrays her at the very end.
https://screenrant.com/fourth-wing-better-job-setting-up-dain-redemption-arc/