Board Thread:General Discussion/@comment-14248438-20160214170304/@comment-14248438-20160214194702

Mabey thay called him "Him" because Alphys called him "Him" and mabey Alphys called him like that ( him him sounds stupit...) because...well because Alphys is Alphys...

Like you said....we can`t be sure unless Toby Fox himself gets here and is like: "Hey, unimportant dudes! Mettaton is Male!"