
I feel like the romantic moments are out of nowhere and they seem isolated. Like there's no lingering of feelings or sexual/romantic passion bleeding into their interactions. Like no heated gazes being crossed, or desire to keep doing things even when the plot is moving. Like they just cut their feelings off too cleanly between scenes.

It feels exactly that way to me. It's as of every kiss they have is strictly on impulse but that's it. We don't see Eustaph holding back his desire or Lan feeling flustered for a long time but this might have to do with the fact they have known each other for a long time. Like maybe they've loved each other for so long that loving as lovers isn't much different from them loving to the point of sacrificing each other's life for each other.
I know it's the translation, but at chapter 102 (recent chap) when Eustaph called Lan "Sis", it kinda gave me the ick. It's hella weird, but considering how that's what he's been calling her for long time in the past, it might have been natural for him. However, if they are lovers, I'd rather they not mention anything about being raised as siblings or whatnot.

Am I the only one that feels zero chemistry between the leads, and no excitement because I don't see the chemistry? Like Ran is kinda boring and it feels out of character for Yustaf to have fallen in love with her. Like they're cute, but I feel like she had better chemistry with lumie.

Personally, the opposite is said for me. I enjoyed the subtle hints thrown every now and again by the author drifting toward a blooming romance development between Ran and Yustaf. Although one could argue I speak with a natural bias out of favor for intricately painted plots and typically slow burn romances. However, be that as it may, their romance was nowhere near sudden. As is expressed in only the most recent chapters detailing the start to their romantic relationship as dating partners. Along with being incredibly similar to one another, they share many habits, memories, etc. For me, this further solidifies as to why they feel like puzzle pieces finally coming together as if it were meant that way in the beginning.
I truthfully felt as time progressed in this webtoon, both leads leapt towards personal growth. In direct reference to this, I sincerely respect the author for not making a patched up decision and throwing either main characters in there without proper backstory, personality or drive. There is nothing more painful than knowing a stories potential could be greater than what it achieved.
As for the circumstances revolving around Lumie and his character, I find him naturally off putting, mainly due to his mild obsession with Ran. I hope that Lumie is given a greater purpose in this webtoon other than being relevant only in the presence and act of following Ran. Lumie has great potential as a character to blossom but given he's not shown a tremendous amount other than as a reminder to a love rivalry, which he has lost, he's not received many chances for readers to see him in a different light. This is main reason behind why I find him difficult to relate to as a character. He's given less chances to portray intricate details and personality traits we'd sympathize more readily with.
I hope eventually Lumie either discovers new passions or is given the opportunity to grow as well, with another person who is suited to him.
End note : sorry for the long note, I just figured I'd insert this here just as a response. It's always interesting to see everyone's different perspectives as readers! (▰˘◡˘▰)
Honestly. I feel like green flag isn't as great as he seems. If he was really a green flag, he wouldn't be putting the mc in danger by hugging and kissing him when he knows that the emperor has laid claim.
I know rightttttt?????
A true green flag would hold himself back. And holding back is so... Hot
HELP the way you delivered that is so cunt. But you really cant expect anything from yaois these days, sometimes characters seem like they have other motive and sometimes theyre just plain out written to act that way
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"is it intentional or is it bad writing?" the reader may never know~