There's not enough information to really know who Lucy specifically is. She could be just a regular reader of fanfiction who took the time to rate this piece as M. Maybe she's part of a fanfiction community where people rate works to help others know what to expect.
It could be inspired by her dreams. People often get creative ideas from their dreams. Lucy might have had some vivid and interesting dreams that she wants to expand on and turn into a novel. She could be exploring new worlds, characters, and storylines that came to her during sleep.
Lucy was the character in the novel. But without more context about the specific novel, it's hard to say more. There could be many novels with a character named Lucy.
I'm not sure what type of Jereff and Lucy Doujinshi you're referring to. If you can provide more background information or context, I will try my best to answer your questions.
Lucy could face isolation. People in the fanfiction world might avoid her, and she could be left all alone. This could also lead to her losing confidence in herself as she is constantly being blamed for something she didn't do.
Lucy may also come across ancient warriors. For example, she could meet a Spartan soldier and be amazed by his combat skills and strict discipline. They could have a brief encounter where the Spartan is curious about this strange girl from another time.
Other dragon slayers could be her potential allies. They might recognize her skills despite her being a princess. Also, magical creatures who are enemies of the dragons, like phoenixes or griffins, could side with her. And some wise old wizards or witches who see her potential might offer their help.