About the secondary characters:
Sorry to tell you that only Bit-na is there. And the Bit-na in the webtoon is a horrible b¡tch that enjoys drama and gossip and only wants to know about other's personal lives but doesn't care about her friends.
The story is solely focused on Nabi and her "guys" from her point of view. We also rarely see our fuck boy lead when Nabi is not around so the story never says anything about his background or why he was acting like that or what his deal is with butterflies (which I hope they can add in the drama). All he ever says in the webtoon is that his family is awful to each other so he thought everyone was like that. THAT IS IT. Even his expressions in the webtoon don't give you the idea of what his feelings are.