Board Thread:General Discussion/@comment-27894627-20160511021754/@comment-27701762-20160511022250

Mettaton, as in the robot, is referred to consistently as "he" by various characters.

Mettaton is actually a robot body inhabited by a ghost, whose name is not stated by the game. This ghost is Napstablook's cousin, and their house is located right next to Napstablook's house. A lot of people have concluded that Mettaton is a girl, or that the ghost was female prior to inhabiting Mettaton, because everything in that house is pink. The name and gender of this ghost is not stated, so it is theoretically possible that the ghost was female until they became Mettaton.