Megumin is characterized by her love for explosion magic. In fanfiction, this trait is usually emphasized. She's also shown as being very determined and headstrong. Her interactions with Kazuma can range from her relying on him for support in non - magic related matters to her getting frustrated with his antics. She often has a cute and somewhat naive side that makes her endearing in the fanfiction world.
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.