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.
Well, Natsu is likely to be the main protagonist, full of courage and a sense of justice. The little demon girl could be a young and innocent being who has strayed from her family or group. There may also be antagonists in the story, such as other demons who want to harm the little girl or humans who fear and misunderstand her, adding more conflict to the plot.
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.
A local witch or wizard could be a main character. They sense the presence of the demon and come to help the girl. They might have their own motives, like they want to use the demon for their own gain or they truly want to save the girl from the demon's clutches.
Perhaps the little girl is a prodigy but is being bullied because of her abilities. Naruto discovers her crying in an alley. He comforts her and tells her his own story of being an outcast as a kid. He helps her gain confidence. They might start training together secretly. The little girl could then use her new - found confidence and skills to impress the other kids in the village, all while Naruto watches with pride.
Lucy could also play a major role. She might be the one to help Natsu adjust to his new life as a girl. Maybe she gives Natsu tips on how to dress and act like a girl, and they could have deeper conversations about what it means to be a girl in their magical world.
There could be a wise old mentor figure. This person might be aware of the girl's actions or the existence of demons in general. They could try to guide the girl, either to stop her from summoning the demon in the first place or to help her once the demon is there.
The plot could involve Natasha taking the girl under her wing. Maybe she tries to protect the girl from those who would misuse her powers. She could train the girl to control her powers, like teaching her self - discipline and combat skills related to using her powers effectively.
Lucy could be a main character. Since she's often by Natsu's side, she would likely be deeply involved in his recovery. She could use her celestial spirit magic to try and protect him while he's injured or search for a way to heal him.
Obviously, the little girl is one of the main characters. And a friendly dinosaur, like a small Stegosaurus, could be another main character. They are the focus of the story as they interact with each other.
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.
Definitely Ruby Rose. Since she's the main protagonist in RWBY, it's likely that Natsu would cross paths with her first. Their personalities are quite different, with Ruby being more innocent and Natsu being more brash, so their interactions would be interesting.