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.
"Criminal Law of the People's Republic of China" article 20 provides for the situation of justifiable self-defense. According to this article, in order to prevent the country, the public interest, the personal, property and other rights of the person or others from being violated, an act taken to stop the illegal violation and causing damage to the person who violated the law is considered justifiable defense and shall not be subject to criminal responsibility. This means that under legal circumstances, individuals can take necessary actions to protect their own or the rights and interests of others without incurring criminal responsibility. However, if self-defense clearly exceeded the necessary limits and caused significant damage, criminal responsibility may be incurred. In addition, defensive actions taken in the context of violent crimes that seriously endanger personal safety, even if they cause casualties to the wrongdoer, are not considered excessive self-defense and are not subject to criminal responsibility.
It's hard to pinpoint exactly where Gojo is at any given time in the manga. His whereabouts change as the story progresses and different scenarios unfold. It could be in a supernatural realm, a mission site, or interacting with other characters.