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.
A 'violin novel' might refer to a novel that has some connection to the violin. It could be a story where the violin plays a significant role, perhaps as a symbol, or it could be about a violinist and their life experiences. For example, it might tell the story of a young prodigy who overcomes many difficulties through their relationship with the violin.