I felt the same at the time, but...
SPOILER
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At some point he seems obsessed with her and I think he was showing it here like a creep. I think he started liking her at that trip too, but well, obsession was still there back then.
OMFG!!! REALLY?! Is it going to cause problems in future? Like stalking, proposal, hating the prince etc?!
No, no, no (ノ≧∇≦)ノ
In his POV, he does admits that he wanted to see her miserable, but as she became regina, he felt much more obsessive with her. And well, at the point I am now in the manga, he likes her. I don't think he admits it, but well, his tongue spoke for himself in one chapter, lol. xD
Well in later chapters he will tell her that he wants to know everything she's doing or saying and who she meets with or talk with. And low-key that he's jealous of 5th prince and her brother lol
Basically he's saying he wants her for himself alone
Side note he never shown any interest in ugly face (forgot her name) but it seems it was an act only....
Ahahah I laughed so much at your describtion of Irene. xD
They made a deal, since she is willing to give him all the authority and basically she just wants to stand as empress. Well, thats what she said, but I'm not so sure about that. it's irrational to be the empress and not want any power. :D
And yeah, the prince is hella jelaous of his younger brother. :D I laughed so hard when he wanted to come with MC to his own birthday party, but his brother was faster. xD
That last part looked kinda creepy in my head like where he was near her ear that just seemed to close even though they are most likely going to end up together but it just didn't feel right to me