Well, start with her appearance. You can describe her having long, flowing hair that changes color based on her mood, which is a cool and unique touch. For her motivation, it could be that she witnessed a great hero sacrifice themselves to save her when she was a child, so she's driven to be like that hero. In terms of relationships, make her have a rival - like friendship with another character. This adds drama and depth to the story.
To create a compelling female MC in My Hero Academia fanfiction, first, give her a unique quirk. It could be something related to nature, like controlling plants, which is not too common in the original series. Second, develop her backstory. Maybe she comes from a family of non - heroes and has to struggle to be recognized. Third, give her distinct personality traits. For example, she could be really shy but super determined when it comes to saving people. This combination of traits makes her more interesting and relatable.
For a female MC in My Hero Academia fanfiction, think about her goals. If she aims to change the hero society from within because she sees its flaws, that's a great start. Her character arc can involve learning how to work with others who have different views. Also, give her some flaws. Maybe she's a bit overconfident at times, which gets her into trouble. This makes her more human and less of a perfect, unrealistic character.