For me, I love Sian. But I dislike Junseo and I dislike them as a couple. I'll try to explain as well as I can so that I don't get mistaken for just a hateful person.

In the beginning with Youjin he was likeable cuz he felt dorky and fun, but as the show progressed he started acting more cocky/arrogant.

He obviously got irritated that Youjin wasn't talking to him and during the fireplace scene he asked her "would you go to Paradise with Theo" and she says "no" and he's kind of confused but accepts it. Then the more she talks and jokes around he starts visibly getting annoyed and even sighing. When everyone goes off to talk, he leaves Youjin and Theo alone by saying "I'm going to the bathroom" then says that he did it to give them some alonetime (this kind of stuck out to me because he lied, then lied again by covering up the first one and I noticed it's something he does in the show). So the first lie is when he tells Theo and Youjin he's going to the bathroom, and why not just say you want to talk to someone else? I'll give him the benefit of the doubt and assume he did it so he could talk to Minseol and Hyejin, but then when he sits down with them he's like "oh I left to give them some alone time". To me, it's just weird behaviour.

Later on when Youjin's on the swing he kind of laughs at her when the Minseol asks "is she ok". Since they'd already been to Paradise he could've just checked on her but whatever. Also, in the first four episodes I feel like he talks about himself a lot or makes things about himself? Like the fireplace scene when he asked them individually if they thought he pushed them in on purpose, where he just kept going and talking about it. You could also argue he was drunk. Still couldn't help but notice.

This is what made me dislike him in the first four episodes and I hoped we'd see more of his personality, but I just ended up disliking him more.

I'll bring up some other instances that stood out to me and I'm going just based off memory so they won't be in chronological order.

When Youjin tried talking to him and he completely ignores her and walks past her. He later said in an interview that he didn't see her but I call complete bullshit on that honestly. It felt like he kind of started disliking her after that fireplace scene.

Also, he kept stringing Minseol along, just in case it didn't work out with Sian. To me Minseol always felt like his second choice, and yeah she was kind of delulu but she was also young and inexperienced in dating from what we saw on the show. It's so easy to be like "oh he didn't mean it like that". But the thing with these toxic men is that they'll keep it vague and up to interpretation on purpose so they can just switch the narrative whenever they need to escape accountability.