The one who left the group is the one we have not yet seen the face of (BaekJun)
He was the oldest in the group and was the main/lead vocal.
The one who shut himself inside the house is one of the youngest team member (so at the time of the crazy accident and all the scandal, this member was barely past minor age, you can imagine the trauma from the press that he is carrying).
The other older member (now, actor hyung), and the other younger member (now, upcoming athlete) both know who the bad guy is, and I think they both still worries that the bad guy gonna approach and hurt MC again.
So, based on the age, these are the members of Dream Boy:
Villain Hyung -> Actor Hyung -> MC -> Athlete Boy -> Shut-in Boy
Either because of jealousy or other inappropriate feelings towards MC, he manipulated MC into some kind of establishment (maybe motel since I saw a bed), forced MC to drink the drug and no idea what else happened.
But as there's business/company reputation to take care of, it was resolved only by making the villain to resign from the position.
And they don't reveal to public about the drug.
The doctor currently in charge of MC, have been researching about the effect of this drug via observation on MC as the doctor have no access to the drug.
The doctor know how bad this drug can affect someone after seen MC.
That's why doctor suggested the hospital he work at to start researching a way to counter this drug before the drugs start running wild in society.
ML seems like some kind of elite member of a certain organisation that's been chasing the maker/distributor of this specific drug.
Either some kind of CIA/FBI or a brazil mafia organisation.
I have few questions and will probably know the answers as the story progress.
The one who left the group was the one who's not leaving his house or the other member who we haven't seen amongst the four member who remained before the disbandment? (thought it was mc but the names different, this is from the chapter where mc's previous co members met)
The one who left the group was probably the one who gave him the drugs? Or one of the four who we haven't seen.
The top is from Brazil and the drugs is from Brazil so there's probably a future connection?
He knows and the manager knows about the drugs and they are probably investigating about it and then he met mc?