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
|消耗品|| 怪物糖果 ● 繃帶 ● 蜘蛛甜甜圈 ● 蜘蛛蘋果酒 ● 奶油派 / 蝸牛派 ● 雪人的碎片|
Bisicle / Unisicle ● 肉桂兔子 ● Temmie脆片 ● 被拋棄的鹹派 ● 狗狗沙拉 ● 太空食品
方便麵 ● 螃蟹蘋果 ● 熱狗 / 熱貓 ● Glamburger ● 海茶 ● 星光雪糕 ● 傳說英雄
不好的回憶 ● 最後夢境 ● Popato Chisps ● Nice Cream ● 垃圾食物 ● Mettaton臉型的牛排 ● 沉默小狗
|武器||棒子 ● 玩具刀 ● 耐用的手套 ● 芭蕾舞鞋 ● 被撕裂的筆記本 ● 焦鍋 ● 空槍 ● 破舊的匕首 / 真刀|
|護甲||繃帶 ● 褪色的絲帶 ● 男子漢頭巾 ● 舊芭蕾舞裙 ● 陰天眼鏡 ● 彩繪圍裙 ● 牛仔帽 ● 心形墜飾鎖 / 墜飾鎖 ● Temy護甲|
|雜項||集點卡 ● 煩人的狗 ● 狗的殘留物 ● Undyne的信 ● Undyne的信EX ● 神秘鑰匙|
|已刪除||門球捲 ● 石糖 ● 南瓜戒指 ● 無淚洋蔥 ● 鬼果 ● 狗麵冰淇淋|