Honestly I agree with you that there's no character development because the story is plot-driven. Everything about who they are and what they do to each other is driven by Almuten being present in their lives. People who've read the novel say that their motivations are better explained here, but I guess the manwha focused more on showing how deplorable the sex was.
I'm not defending it, I'm just explaining why I think it makes sense to show it like this. In a war, all the unpleasantness appears, there's no way around it. Its themes are really dark. Hopeless, despair, end of the world. All the espers have mental issues. Yugeon does too, tbf. Yugeon felt attached to Shinje because he's a warm living body. If it's too heavy for you, let it go.
FORCED and I mean FORCED myself to read the first 30 chapters because I liked the design of the ml and MC. But the rape? I literally had to grit my teeth because of how uncomfortable it was. It's bl and I know non consensual sex is common so no matter how awful it is, lots of good stories have it so you have to put up with to get to the good bits. BUT ITS ALL RAPE. Also I know it was a harem but usually it cuts down to just the ml sooner or later? Not this. They literally talk about and treat him like a toy. "Isn't he ours and not just yours?" "I've been thinking about fucking him" EW EW EW EW what the fuck! Literally nothing enjoyable about this it's LITERALLY STOCKHOLM SYNDROME. He's just gotten attached for some reason? Lots of people say codename Anastasia is Stockholm syndrome and roses and champagne. NO! THIS SHIT IS!!!!! Then I fast forward like 40 chapters thinking ' it can't be this bad all the way through " AND IT IS. DOESN'T GET BETTER. THERES LIKE NO CHARACTER DEVELOPMENT. I'm usually bot this hateful about manwhas and I can put up with a lot but this is seriously disgusting. Author you have a lot to say for yourself