Board Thread:Questions and Answers/@comment-39106664-20190420011352/@comment-35757757-20190903231923

Now here’s a thing on Mettaton on TV Tropes (copy and paste)

He even goes through a pronoun change— Undyne, his former neighbor, only calls him "they" when you call her from his house, Napstablook's house, or the farm, implying that "they," "them" and "their" were the pronouns he used before he was able to transition into a robotic body. In all forms of his robot body, however, he goes only by "he," "him," and "his." Since Mettaton disappeared without a trace and didn't keep in touch, there would be no way for Undyne to know about the pronoun change. Another possibility is that ghosts, being incorporeal, go by "they", "them", and "their" by default, since all ghosts in the game are referred to this way, and only begin to identify with a specific gender once they've inhabited a body.