My theory, though it doesn’t make much sense, is that Aquila planned for Uriel to become the duke. Let’s say that during his time as a knight, something happens to him while working with Aquila. I’m guessing they were close, but perhaps more so than we think. And maybe Uri isn’t as incapable as we think, he used to be, but as a knight he probably had to face the real world at some point right? And so he gains skills like insight, and understanding how to handle things. However his way of doing things is very different from his sister’s. As Lamia is allied with Ethan, since apparently if they grow up together she dies, he distances himself from her but keeps tabs and allies himself with Uri once they meet. So either Uri is allied with or working under Aquila to protect Lamia in some way. Also, keeping her away from the position of duchess would likely save her from dying as it seems that also leads to her end. So as a knight, she won’t be as big of a target as she would of been if she was directly involved in politics as duchess. Uri’s keeping secrets or hiding something about Aquila, and Lamia, not knowing the whole story wants revenge because she loves her brother and believes it to be the right thing. And the true enemy is not Aquila but something even worse, and her prince friend is either evil, an ally, or involved in some way. Motivation for everyone has something to do with Lamia, and preventing her death. Lamia marries Aquila by the end, or maybe he sacrifices himself for her happiness and she spends eternity alone or marries someone else, and if they’re both alive Aquila becomes king/emperor/whatever (but that’s a given, and maybe something else he needs to do to protect Lamia, gain absolute power that he never really seems to have aimed for up too now)I’m probably totally off though, this was just a theory that came into my mind, especially since we don’t know much about what’s happening at all.