/jp/ - Otaku Culture

There is a fan theory that Maribel and Yukari Yakumo are actually the same person. The basic idea of this theory is that when Maribel sleeps she dreams that she is Yukari and when Yukari sleeps she dreams she is Maribel. Because Yukari is nocturnal and is known to sleep during the day neither of them would normally be awake at the same time.

Maribel is strangely able to cross the border into Gensokyo while dreaming. Some fans go further with this theory to suggest that Gensokyo itself was created in Maribel's dreams.

紫の夢の世界が外の世界という噂も。
>Rumor has it that she is a youkai from outside, and she simply comes to here for play. Indeed, she may spend the winters and the daytime, when she claims to be sleeping, in the outside world, and some even say that the outside world is Yukari's dream.
http://touhou.wikia.com/wiki/Perfect_Memento:_Yukari_Yakumo