The Mystery Key is an item sold by Bratty and Catty. Bratty and Catty describe it as "????? Probably to someone's house LOL." It unlocks the house adjacent to Napstablook's home.
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 displays:
- You used the Mystery Key. Mettaton pretends it isn't there.
Bratty and Catty mention how Mettaton said that being built was his own idea and how he considered Alphys to be an old friend. 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".
- Dear Diary: Shyren's sister "fell down" recently. It's sad. Without her sister to speak for her... she's become more reclusive than ever. So I reached out to her, and told her... that she, Blooky, and I should all perform together sometime. She seemed to like that idea.
- Dearer Diary: I like to buy a new diary for every entry I make. I love to collect diaries.
- Dearest Diary: Our cousin left the farm to become a training dummy. That leaves just Blooky and I. Blooky asked me if I was going to try to become corporeal, too. They sounded so... resigned... come on, Blooky. You know I'd never leave you behind. And besides... I'd never find the kind of body I'm looking for, anyway.
- My Darling Diary: I met someone... interesting today. Last week I posted that advertisement for my Human Fanclub. Today was our first meeting. Only one other person came. Honestly, she's a dork. And she's obsessed with these awful cartoons. But she's kind of funny, too.... I want to see her again.
- Diary... My dear: My diary collection is going fabulously. I have like five now.
- Dear Diary: She surprised me with something today. Sketches of a body that she wants to create for me... a form beyond my wildest fantasies. In a form like that, I could finally feel like... "myself." After all, there's no way I can be a star the way I am now. Sorry, Blooky. My dreams can't wait for anyone...
- 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