Some Niragi fanfics could be about his relationships with other characters. Maybe a fanfic where he forms an unlikely alliance with someone he initially hated. This could be really interesting as it would show a different side of Niragi, a side that might be more complex than his one - dimensional 'villain' persona in the original story.
One possible reason could be that she's different. In many fictional worlds, being unique can make a character a target. Maybe her appearance or her abilities set her apart in a way that makes others jealous or feel threatened.
Teaching the bullied kid self - defense skills, not in a violent way but more about how to be assertive. They can firmly say 'no' to the bully. Also, encourage the kid to make more friends. Bullies are less likely to target a kid who has a strong group of friends around them.
The girl could find allies. Other kids who are against bullying can form a group with her. They can support each other and work together to raise awareness about bullying in their school or community. This way, not only can they protect the girl from further bullying but also prevent others from being bullied.
The theme of survival in the Borderland also plays a big part. They are both trying to survive in this dangerous world, and their relationship can either be a hindrance or a help in that. In some fanfictions, they have to rely on each other despite their differences to make it through various deadly games.
Some fanfictions may have the theme of resilience. Romano could show great strength in the face of bullying, not letting it break him completely. There might also be a theme of finding one's voice. He could start off being unable to stand up for himself but gradually learn to speak out against the bullies. And the theme of change is possible too. The bullying experience could cause Romano to change his behavior or attitude either for the better or the worse.
One common theme could be Natsu's struggle to overcome the bullying. He might initially feel alone and defeated. Another theme could be the discovery of true friends who stand up for him. For example, his Fairy Tail comrades could find out about the bullying and rally around him, showing the importance of friendship in difficult times.