Serie became aware of her feelings kinda "early on", like 2/3 ways of the story, but since the MC is like a celeb in their town, there it was absolutely no time to be alone and confess, and there it was also plenty of world level crisis in between to hamper her romance
The last lines are Serie embracing the mc and finally telling him her feelings in a very sweet scene.
The problem with the end, is that too many characters were left hanging. The yuusha betrayed his kingdom (and technically, every kingdom), but we don't really now what will he do afterwads, I'm not even sure if he ran with who he wanted to due to how fast his circumnstances flashed
Also, romantically, the yuusha and the mc both have the same circumnstances : they only love one girl, but there is a total of 3 girls after them; in both cases the third girl is an "outisder" and sort of realizes that their chances are non existent, and sort of accept that they have to give up. In both cases, the second girl is sort of aware that being the main wife is impossible, so they are better of aiming at everyone together, but we never got to know how that played out.
And well, since both the yuusha and the mc are living outside the laws, I don't think that "ethics" are a decent reason to refuse.
.... Let's hope they expand on these issues on the published version.