Whose house is next to Nabstablook's house?

I've looked it up, but to no avail. I read that you need the Mystery Key, but I still don't know whose house it is.

2 Answers

The house next to Napstablook's belongs to Mettaton.

You can buy the Mystery Key from Bratty and Catty for 600G.

Also:

in the True Pacifist ending, you can wander around the game world after beating the last boss. Mettaton will be found hanging around outside his house in his Mettaton EX body.

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Nabstablook's neighbour is his cousin, the ghost, mettaton. Most people that have watched a playthrough of undertale or played it themselves would know mettaton as a robot who later turns into a different, more human like, robot. If you buy the key from bbratty and catty you see that there are 6 journals or diaries about how mettaton meets the scientist, Alphys. The journals/Diaries also explain how close nabstablook is to mettaton. that is where the 'cousin' theory comes from.

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