In a fanfic, Moxxie's accent could be a central part of a romantic storyline. Suppose there's a character who is initially put off by her accent but then gradually falls in love with her as they get to know her better. Her accent becomes something endearing and special, and they have cute exchanges where he tries to imitate her accent playfully.
The main characters are Striker and Moxxie. Striker might be seen as the more dominant and aggressive one. He has his own goals and ways of doing things in Hell. Moxxie, who is associated with I.M.P., has his own set of friends and a life that gets intertwined with Striker's in the fanfic, creating a lot of interesting interactions and storylines.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
It can be depicted through her dialogue. For example, using specific words or phrases that are characteristic of her accent. Like she might say 'ain't' instead of 'isn't' or use some unique local slang.