Board Thread:General Discussion/@comment-27894627-20160511021754/@comment-28247397-20160527214020

I know I saw a theory floating around about the previous fallen humans that discussed the yellow soul and how information about it can be found near Napstablook's cousin's house which indicates it belonged to a girl named Suzie. Seeing as the color associated with Mettaton's soul/attacks is yellow, so some people concluded that Mettaton is the ghost of the yellow-souled Suzie. What that means for his gender is that he was potentially a fallen girl human who became a genderless ghost who then became a gender nonconforming male robot.

There are many, many headcanons and variations of this (admittedly somewhat convoluted) theory, all of which get at the point of Mettaton being transgender in some form, and at the very least, highly gender nonconforming.