That is Hosik and Sungyeon in their second life. Their first life was when Hosik died of tuberculosis (?), where he was coughing a lot. In a lot of multiple lives stories, they usually don’t completely remember but since Sungyeon must’ve been so in love and hurt by Hosik he remembers to a certain extent.
Okay so this is just my theory.
Hyung remembers because the pain of losing hosik was so much that he carried those memories with him to his next lives.
That's why he gets so possessive, paranoid, manipulative when it comes to hosik.
Hosik died, though he felt grief for leaving hyung, he left kinda fulfilled coz he saw him one last time before he really died. So in some way, he has lived a happy life.
In a way you can say that Hyung's soul is wounded and is continually bleeding. That's why he remembers it coz he hasn't been healed.
Hosik's soul has a scar, that's why he will always fall for hyung no matter the era but he doesn't remember.
That's just me, though.
Am I the only one confused like is that hosik or not? and why doesn't he remember? Can someone plz explain