And I still don't care about his backstory. My only question is, he kissed the other dude ...

It’s because he kissed him back in the end! He only kissed him to show that his tongue had a T mark for the main guy to believe him.

Funny thing. How did he have a T mark? Did he kiss someone with a T mark ? If so why the heck did he take the F mark back and kiss the main guy for the D mark?
Weird

Its because Jinsol thinks having to lie and told truth is a privilege unlike saying only truth. Do you remember ehat happened to their apartment where there is a fire? Jinsol knows that the police will questioned him. I think he need to tell what caused the fire and at the same time make them the victim

You are right, he has to be able to tell the both lie and the truth so it won't hinder the story he is planning to tell the police

I'm just still confused about a) how kissing can pass the mark between two people, and how that all works. And b) how he even knows that it's a thing, like how did he even come up with the idea that kissing can give you someone else's mark.

Same tho! Hope it will get revealed haha

yeahh he kissed the main guys crush!! idek though maybe he wanted to be able to tell both lies and the truth

TRUEE HAHAHA