Board Thread:General Discussion/@comment-27873549-20160419202426/@comment-27701762-20160419215246

The wiki does indeed say the house belongs to Napstablook's cousin.

The diary entries in that house describe the journey of said cousin, who is close to Napstablook, and at some point befriends Alphys ("Honestly, she's a dork. And she's obsessed with these awful cartoons.") This monster also has dreams of being a star ("After all, there's no way I can be a star the way I am now."). Alphys shows them plans for a body that she can construct for them, which gives them the ability to leave the farm and pursue their dreams.

The diaries absolutely describe Mettaton, and not Gaster.