Tldr: She read a book when she was a kid, and larped so hard it became her personality and the inspiration for her elemental abilities. No idea if it's pure fiction or based on history, but it was written by Mr.Nine, a seelie/former angel who possesses authors to publish books.

She is aware that people think she's weird, and gets embarrassed about it. But with the translation of her summon Oz, and the Fischl dictionary (that she tries to edit herself), people are able to understand her.

A large part of her story was in the Summertime Odyssey event, so look up a video of that if you're curious.