
Spoilers
ML is hyung but has no memories from other life. They were in love but ML hurt MC. Friend of MC sister wrote novel where there are supporting characters so when they die (some building colapsing) they migrated to world from that novel, Hyong is older, MC now has younger brother. Only MC didn't die, he hugged hyung's body for days, slowly daiying and his migration was incomplete, that's why he has health problems that noone can explain and nothing can cure him.
In few chapter MC should have a dream about building colapsing and he will know that there is no world he can go back, so he can stay with ML but his health will get worse and ML will pull more and more crazy shit
I’m like 99% sure the ML is supposed to be his hyung from his past life, if that wasn’t alr a little obvious LOL. It also explains why his “character” is different than how MC remembers it. But if I’m right I wonder how ML ended up in the book too ?