Maribel's nickname is "Merry" or "Mary." (Katakana: メリー) She also complains about Renko never calling her by her full name because it's hard for Renko to pronounce.
As Maribel's name is written in katakana, she is not of Japanese ethnicity. This is confirmed in Neo-traditionalism of Japan in which she says she has no relatives in Japan.
Due to the similarity in appearances and abilities (Maribel is even speculated to be manipulating boundaries in Changeability of Strange Dream, and demonstrates abilities that would fit that in Dateless Bar "Old Adam"), Maribel is often speculated to be related to Yukari Yakumo. This theory is only fueled by the following: that Renko likens Maribel to a youkai in Neo-traditionalism of Japan, that at the end of Magical Astronomy, Maribel considers visiting the moon through its reflection on the surface of a lake, which happened to be exactly what Yukari did during the events of Silent Sinner in Blue, that Maribel has been known to travel through time, as can be seen in Perfect Memento in Strict Sense and Neo-traditionalism of Japan, that "Yukari Yakumo" is speculated multiple times not to be Yukari's original name, and quite interestingly, that when asked about all of this evidence, ZUN simply stated that "In the past, there was this man named Lafcadio Hearn". Lafcadio Hearn was one of the earliest authors to write on youkai in the English language. That is not the interesting part, however. The interesting part is that he naturalized as a Japanese citizen after living in the country for a while, and chose his name to be Koizumi Yakumo.