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.
For a beginner, 'Gojo's First Adventure' is a good choice. It's easy to follow and gives a great introduction to Gojo's character. It has simple language but still manages to capture the essence of Gojo's cool and powerful nature.