[Kanon]

Originally Posted by Shadow5YA

Spoiler:

The Nanaki comparison only makes Cloud look even worse.

Keep in mind that Nanaki has also been experimented on by Hojo, and that despite that, he still could not see her.
"Sensing" her is also questionable if you pay attention to his body language. He raises his head, but doesn't turn toward where Aerith is supposed to be.

There's also multiple occasions where Aerith disappears when Cloud isn't paying attention.
You can see here that when Cloud is looking in Nanaki's direction, Aerith is there:


But then when he's looking at the colorless materia in his hand, Aerith is gone:



If she were a ghost, she would be there regardless whether Cloud is paying attention or not.

Spoiler for Ending:

Nanaki literally says "Aerith" right as she walks by him.

And Aerith is still there after the Tiny Bronco takes off and Cloud is long gone. The shots where she disappears is to make it clear the other party members don't see her, that's all.