Okay, I disagree with some of you guys lol. Kat and Sophon are in an OFFICIAL relationship. They have been since the beginning. Didn't you guys pick that up?! They are in a long-term, grown-up relationship, and it's even known to Kat's parents. They're practically engaged! That's why I was so bothered by Kat and Paul being so close (although I do love them together haha). And obviously, Kat and Paul have feelings for each other. Paul realizes it, but he's not telling her. And Kat has not realized her hidden feelings yet. You can tell that she definitely admires Paul though.
And another thing I disagree with: I don't find that Nun and Sophon are flirting, nor do I think that they are anywhere near being in love (not yet, at least). I am sure that Sophon is exclusively committed to Kat (at the moment). Nun/Sophon's intimate relationship will be an accident in the future, but as of now, I believe that Nun is only getting so close to Sophon to keep an eye on him because she doesn't completely trust that he is a gentleman, because she loves her sister to death and wants to watch out for her. And those 2 have hated each other from the beginning. I took it as...Nun was just the annoying little sister of his girlfriend, and that was it. Although now, we see progress in their relationship: they can stand each other lol.