Mystery Key

An item purchasable from Bratty and Catty to the right of the MTT Hotel for 600 Gold. Catty and Bratty describe it as "????? Probably to someone's house LOL" and it in fact unlocks the house to the right of Napstablook's house. The item is consumed to open the door, and has the description of "".

Main Story
The key unlocks some side-content that does not impact the Ending of Undertale. It is revealed to be the house of the ghost that inhabits Mettaton, and reveals much of Mettaton's backstory.

It is explained that the soul that Alphys put into Mettaton is in fact Napstablook's cousin and neighbor, and wanted to become corporeal, but that he'd "never find the kind of body [he's] looking for" He hosted a "human fanclub," to which only Alphys showed up. She ended up sketching and showing Napstablook's cousin an ilustration for a body "beyond [his] wildest dreams." It is presumed that this was an illustration of Mettaton, to which he agreed to be put into, due to his desire to become a star.

Trivia
If Undyne and Papyrus are called while in the house, Papyrus mentions that his headcanon for the ghost's name is "Happstablook the happy ghost."