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 cold smile or a hot haha was a kind of opposite smile. A cold smile was a smile that contained contempt, ridicule, helplessness, anger, and so on, while a hot haha was a kind of passionate smile. The cold smile and warm laughter were fake and pretentious, making people's hair stand on end. The word 'narrow-minded' was used to describe a person who was careless and big-hearted. Sometimes, it also implied that he was simple-minded. This word was usually used to describe a person's attitude of letting things that were related to their own interests develop in a bad direction.
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.