@Mattitude12
The ghost girl can hide her presence and choose when she wants to be seen. When the balloon flies away you see her get all surprised and she fades away into black. In chapter 22 you could see her shaded in black as well, which is why our college MC didn't react to her sitting on his lap. But this is just my theory. It could be that the author is just being inconsistent.
@FirstDayRegister I think he can see her sometimes when she's trying to scare him, but most of the time he can't. Maybe there are conditions to seeing them. Like, when they've got murderous intent? When someone is on the verge of sleeping? When the ghost is in the particular place where they died? It can be anything.