>>527518163
Chara was surely old enough to be capable of saying "I'm a boy/girl btw". And I don't know why monsters would assume this when they themselves have gender.
>the book just referred to humans in plural
Which book?

>>527518168
>mettaton's pronouns don't change
There's an optional phone call where Undyne they/thems Napstablook's cousin who left and whom she can't remember the name of (and who is clearly Mettaton).
All the ghosts are consistently they/themmed in Undertale even when it makes the sentence awkward.
>Mad Dummy (don't think pronouns are used but its very masculine)
IIRC they/them plus occasional it/its in battle (which is normal even for gendered enemies).
Mad Dummy reads masculine but I think Mettaton's old house would read feminine without the context of robot Mettaton.