The the brother guy is probably the body guard guy
Actually no, it's someone else as they don't look similar that much at all, but we don't know yet as, it's presumed the brother is dead, I don't know actually on it.
Yes he’s dead. Also that boy wasn’t his real brother, he was the son of that woman that the dad started dating. All of them died. And the ML developed that disorder when a person has two totally different personalities (forgot how it’s called in english). When he falls asleep his second personality wakes up, and that version of him is very soft and vulnerable. Things will start happening between that version of him and the MC. The first (rude) version won’t know about it for some time.
Yeonwoo have gone through pretty bad upbringing in terms of financial and economic status, they were poor and his parents neglected almost like abandoned him, an individual whom he calls brother,but I don't know who is he exactly so he protects yeonwoo from his mom tantrums, then one day came upon as yeonwoo was making ramen for himself as he was hungry until the somehow the stove flame break down in causing fire on home as he tried to wake his father but he didn't woke up, I think he was intoxicated or asleep then he was inside the room where fire had happend no one went to help nor took initiative to look inside the apartment complex until the so called brothee came and rescued yeonwoo.
These all I think I could understand from reading raw though also before this he kissed jihan and was crying so much that jihan felt unable to avoid it then somehow the manager hyung comes up and tells on claiming I guess on yeonwoo's split personality aka dissoactive identity disorder as he saw yeonwoo being usual his behaviour demeanor then one day he saw yeonwoo crying and almost strangled manager until he backed him off, somehow the incident which fire broke out did had being one of the reason him getting affected the most, I think the brother may have died or something else happend. Maybe I wrong but I think he was adopted or someone brought him back as that's how he gets educated and become star as he have access to a home with stability and everything but his mental health gets untreated I guess idk maybe I am wrong.
Maybe I am wrong with what I intrepreted here, I do apologise on if I did make any mistakes here, as the raw was not in english so all I did was intrepreted the events from reading the chapter by visual images .
But I do hope yeonwoo understands his behavior actor the first personality I guess towards manager and jihan , as mental disorder like D.I.D is an extreme mental condition which could lead to extreme and severe damages if left untreated, so I do hope the portrayal is sensitive and the manner of how an extreme conditions like split personality should be portrayed upon.