I'll just give you a short version.
From the title itself, Seju was projecting himself into Sejin, not his sister. When he decided to help him, it was himself he saw in him and not his late sister. His actions was constantly resulted by that until the climax. So he was so afraid if Sejin sees what he actually does in his work. Especially since he was a doctor before.
To be honest, that part also made me very emotional. For me, it was a healing story; a self love and forgiving oneself kinda story, not just romance.

Okay… I don’t think we realllyyyy need to know what’s happening inside the gang (ngl I’m confused af) but more so how it’s changing things for Seju. I think the author might be cornering him. First, he ran from home and the hospital cause of his lil sister, somehow ended up in the underworld to take revenge I think (?) don’t remember well, and then thanks to the kid he lowkey liked being home until the recent grief and now noticing the kid caught feelings he no longer feels safe there…SO he goes back to the gang where it’s familiar but gets faced with the cruel reality. Smoke and mirrors chatter for him as he realizes not even there is safe nor where he wants to be….so this might just be a plot to create a turning point and get him back home… ┗( T﹏T )┛idk man I’m just trying to understand