What is with these bad takes/lack of reading? It's just... Failure to read.
Haejoon is almost explicitly able to see Miyeong/the ghost. The paneling from the previous chapters all but confirms that yes, he sees her and is glad to be able to see her.
And no, she doesn't show up only for Haejoon. Eunyung has very explicitly also seen her hanging from that bunk bed, hence why we literally read him say that he sees the ghost. He says it. She is visible. Read the words in the comic.
Like, holy crap, reading comprehension.
She doesn't only show herself when Haejoon isn't there. She is still there even when Haejoon is there. Look back at the previous chapter, two back, where Haejoon, Juwan, and Eunyung are in the room. Haejoon sees Miyeong and then falls asleep because it's familiar and comforting to him.
Three comments here solved by reading...