My favourite thing about this series was how Yona had to learn there's a world she doesn't know, that she has to fight for what she thinks is right, and that there's no easy answer to every problem. The manga starts with Suwon killing Yona's father, which completely breaks her. But instead of making Suwon into a one-note villain, the story (at least in the first part) presents him as a complex character who had every reason to kill the king, even if it meant betraying Yona's trust. It's a difficult situation because for Yona he is a "villain", but for the people who suffered under her father's rule he's a good king.
My problem with the manga comes in the sencond part. Yona has matured into a responsible person and she has people she can rely on, but suddenly every conflict is solved through talking, and Yona is always right. There are situations, especially in war, when common sense does not apply in real life, and there are leaders who cannot be reasoned with, which is why fighting is inevitable. However, in this manga, Yona always gets to choose the easy road that involves no bloodshed, which suddenly makes Suwon seem like an incompetent ruler, even though he studied his whole life to become king and other characters also acknowledged his potential. Forget about experience, education and knowledge, turns out Yona was always destined to be queen, even though the story made it seem like she was happier travelling with Hak and the dragons, helping others. This will be a controversial opinion, but Suwon should have stayed king, and should have atoned to Yona by being a good ruler who helps the country. It would have been a much more complex and realistic ending, Yona becoming queen to me was cliche and the easy ending.
Big spoilers here
Nina loves Sett, she even told him in the latest chapter. She refers to Az as her first love, and he is important to her, but not as a lover. Again, pretty big spoilers here, but the important reason why we had to know who Nina loves is because that person has to kill her when her powers go out of control. So even if Nina loves Sett, their time together is limited.
Also, you can read this manga for free on the official Km@nga app (Kod@nsha's manga app). You can get points by scrolling through a given chapter each day, and you can also watch 3 ads. It doesn't take up much time and you can read anything on the app for free if you have the points or daily tickets. Finished series/some shounen titles can be mostly read with daily tickets. So you can support the author legally for free.