I won't spoil the ending, but something Spike says at the beginning of the last day and another character's response was (for me at least) a bit of a clue. Also, the guest having a vowel in their name is more accurate than it might seem.
GoedWare Game Jam Halloween Part 2 ๐ป(+ prizes ๐) ยป Entries ยป Uninvited Guess (GoedWare Halloween Jam 2)
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I have finally uploaded a full walkthrough including the identity of the Uninvited Guest at the very end!
https://itch.io/t/5507923/walkthrough-spoilers