Board Thread:General Discussion/@comment-26006155-20181123023529/@comment-32182236-20181123155733

..I have a completely DIFFERENT proof that Napstablook is not human!

You see, they have a cousin, who we know is now Mettaton. The important thing to take note is that they're COUSINS. So if Napstablook was a human, Hapstablook, and by extension, Mettaton, would have to be a human as well, and around the same age as Napstablook (in terms of millenia, a few decades isn't much of an age gap) But Mettaton CAN'T be a human, because if he WAS a human, his SOUL would appear with the other six, and Asriel would have the power of eight SOULs, not seven:He wouldn't need the monster SOULs, his seven would be enough. And we can't just say that Mettaton was absent, because he was there with the others when they told us to stay determined (He was the one that told Undyne and Alphys to just kiss already for dramatic tension.)

Therefore, Mettaton CANNOT be human, and thus, neither is Napstablook, since they're cousins.

We don't even need to make assumptions this time! Anyone could argue that maybe that "best customer" of Napstablook's is some random monster that's now dead, but this looks to be a solid proof. (Though maybe I missed something too..)