Well for one, ghost girl is confirmed dead girlfriend soooo.... yes she is a ghost.
He sais many times that she's just an illusion. So we don't really know her true nature. And, every time Jimin arrive she disappear, so she could really just be an illusion created by the guy.
But like, if its an illusion, why does Jimin feel her too? Like in the last chapter
Do you mean the part when he kneels beside the bed? If yes, I think the author wanted us to understand that he's slowly taking the dead girl's role in Hyunwoo life.
There are mental disorders where people see hallucinations and believe that they are real people. They are able to touch them, feel them, etc.
Yeah, but as far as we know, Jimin isn't mentally ill.
Why is everyone hating on the ghost girl? We don't even know if she's really a ghost! Unless I missed something we don't have enough profs. But if she is an actual ghost, then yeah, what a pain in the ass -_-