I have not read the book this story was based on, so my opinions are strictly based on the show.
First of all, Khai was not a good friend to Third. So, I really don't understand why Third was so passionately in love. Again, I don't understand why Khai fell in love with Third. Were the feelings always there? Or it was because of the video? Did Khai even like him as a friend first? Or he just tolerated him? It felt like the plot was rushed halfway. Both were incredibly toxic towards each other even though no one is entitled to the way how the other feels. After they start dating 'officially', Khai gets kissed without consent/is assaulted at the bar. But Third creates a ruckus without even giving a chance to Khai to explain. What kind of a boyfriend wouldn't believe that you are capable of getting assaulted? And after that Train scene happens (this is the only scene that almost brought me to tears thanks to OffGun), Third says "why didn't you tell me?". You threw a literal glass bottle at Khai when you saw him coming towards you to talk?
The OST was awesome. Un and Two were cute. You can give this show a try.