Mystery Key

The Mystery Key is an item sold by Bratty and Catty. Catty and Bratty describe it as "????? Probably to someone's house LOL." It unlocks the house adjacent to Napstablook's house.

Additional Uses

 * If the Mystery Key is used on the Mysterious Door, it reads the following text:
 * You used the key, the lock clicks..... as the key failed to fit in the hole.
 * If used during the fight against Mettaton EX, the following text will read:
 * You used the Mystery Key. Mettaton pretends it isn't there.

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

Trivia

 * If Undyne and Papyrus are called while in the house, Papyrus dubs the ghost who Mettaton once was " HAPPSTABLOOK, THE HAPPY GHOST ". Alongside this, the name of Mettaton's house in the game files is room_water_hapstablook.

Mysteriöser_Schlüssel Мистический Ключ