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, however there has been cases in which both have been awake at the same time as Yukari has been known to be awake during the day as well. Maribel is strangely able to cross the border into Gensokyo while dreaming. Maribel's clothes are also very similar to Yukari's clothing from Perfect Cherry Blossom, and both wear red, white, and purple in the various outfits they have (the colors of the visible spectrum border). Both characters have blonde hair and (in some depictions) purple eyes, and have abilities related to boundaries. Some fans go further with this theory to suggest that Gensokyo itself was created in Maribel's dreams. However, none of this is officially supported. (In "Perfect Memento in Strict Sense," there is an entry about a mysterious memo by a person who believes she is wandering in the bamboo forest while she is dreaming. The key phrase "I'm gonna tell Renko about it when I wake up" is a clear tip-off to whose memo this is. This would seem to disprove the preceding theory, since the memo's author would be Yukari, were it to be true.). Other fans and fan games suggest Maribel actually became Yukari later on.
Another theory linking Maribel to Yukari stems from the fact that there has never been an official romanization for her name マエリベリー・ハーン (Maeriberii Haan). It is possible that the ハーン in her name does not translate to "Han" but "Hearn", as in Lafcadio Hearn the Western author of Japanese Youkai horror stories. Lafcadio Hearn was known as "Koizumi Yakumo" after he gained Japanese citizenship.