Spoiler: To answer everyone's question, the ml and mc did did end up together
The ml was the mc's anchor, his stone that keep him grounded. Without the ml the mc would probably already committed su"cied.
The ml did end up remembering their past. And the sad thing? He thought the mc also dies together with him. When he found out that mc is still alive there, he cried. He cried in anguish for mc, he cried cause he hated himself why he left mc there in that underground, he cried why he died and left mc alone that place, barely alive. Ml hated himself why he died, he should be there in that underground accompanying the mc, but here he is; alive and well with almost no memory from their past live, while his lover is still suffering.
Did the mc never resented ml? Yes he did, for dying and leaving him alone that place. For protecting him in that accident living him half alive and in this situation.
The novel is full of angst. I remember just feeling sorry for mc and ml. The manwha's plot is just barely starting.
And what's fucked up? His brother in this world? Was also the same person, his sister in the another reality. With all memories intact. His friend the one who came from abroad? Also the same friend from another reality with his memory intact. Mc's brother (sister) and friend never let mc know for their selfish reasons. And from what I remember they also died in that accident in the train.
I'm not gonna spoil too much, but I never like mc's brother (sister), and u'll know why

I read the novel long time ago. My memory is not that good woth details. However here's some that I still vaguely remember
The reason why he's sick
- We all know that he came from a different reality, and the mc and ml have met there (if you all guess it).
And if you guess it base on some flashbacks and some glimpse, the mc got into an accident. To be precise he and the ml got into an incident where the underground terminal of a train station collapse
What's worse is that the ml died protecting mc. But the mc is still alive. Yes you heard it right. The mc is still alive. Barely. Half alive half dead, with the ml's body on top of him and on him are the collapse surrounding.
Essentially he's soul transmigrated while he's body is still alive, there underground. That's why he's body is sick, he's having this headache, barely walk, cause he's soul is transmitting the pain and sensation from his alive-dead body.
It's not that accurate but yeah.