There could be original characters created specifically for the fanfiction. For example, a magical creature that befriends Natsu's little sister and helps her on her journey. But the core Fairy Tail characters usually play important roles as well. They are the ones who provide the familiar backdrop of the Fairy Tail world that the story is set in.
One interesting plot could be her going on adventures with Natsu and his friends. She might have unique magical abilities that help the group in their quests.
I'm not sure who exactly Natsu is just from 'natsu reads fanfiction'. It could be a normal person who just enjoys reading fan - made stories. Maybe it's a nickname for someone within a particular fan community.
In this fanfiction, it may be that the little demon girl is lost in the forest and Natsu stumbles upon her. At first, she might be scared or hostile towards him, but as time passes, they develop a bond. Natsu could then take it upon himself to help her find her way back to her own kind or a place she belongs, all while they encounter different challenges like dangerous spells or other unfriendly beings in the forest.
Natsu might be a powerful wizard with a big heart. The little demon girl could be an outcast among her kind, which is why she's alone in the forest. There could also be other side characters like Natsu's friends who might later get involved in the story when he tells them about the little demon girl.
In the 'natsu went mia in war fanfiction', Natsu is presumably a central or important character. He could be a hero - type figure who was expected to play a significant role in the war. It's possible that he has some special abilities or traits that made his presence crucial. Since he went MIA, the whole story might be about his friends or allies trying to find him or dealing with the aftermath of his disappearance. It could also be that Natsu is a character with a complex backstory, and his going missing in action is a key event that will unfold and reveal more about his past, his relationships, and the overall world of the fanfiction.
One plot idea could be that Natsu's little sister discovers a new and powerful form of magic that no one has ever seen before. She then has to learn to control it while also facing off against an evil wizard who wants to steal her power for himself. Another idea is that she goes on a journey to find out more about their family's past, which leads her to uncover long - lost secrets and reunite with long - lost relatives.
Her character can be developed by giving her unique magical abilities that set her apart from others. For example, if she has the ability to manipulate time, she would have to learn how to control it without causing chaos. Also, putting her in challenging situations like facing powerful enemies alone can force her to grow.
Well, Romeo is often depicted as a passionate character. In the context of a fanfic, he might show intense emotions towards Natsu. Natsu, on the other hand, could be shown as strong - willed yet having a soft side when it comes to Romeo. But it really depends on how the author interprets them in the fanfic.