For those who don't get it, Mettaton is actually Napstablook's cousin. His/her house can be found right next to Napstablook's, and most of the stuff you find inside the house confirm the fact that he/she is Napstablook's cousin

More spoilers ahead;

The house next to Napstablook's house used to belong to Mettaton. You can purchase the key to him house from Catty and Bratty, and from there you can find diaries chronicling his time with his cousin up until he met Alphys.

It also reveals that one of the dummies (or at least the ghosts possessing the dummies) is also their cousin, and Shyren had a sister and they lived on the farm at some point.