Board Thread:General Discussion/@comment-31536324-20190117214835/@comment-31536324-20190217134100

"Minmago: They're not a monster because they don't have the one important thing that monsters do, physical bodies. They're rather body-less souls, that work the same way as the lost souls.

- This is all that I can answer to. That is what I got stuck on arguing with them about."

Nabstablook DOES have a physical form though, we physically try to push him out of the way in the Ruins. If anything Ghost Monsters are just monsters that can control the tangibility and visibility of their bodies, thus allowing them to possess other things.