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.
Extreme speed and rapid speed both referred to the concept of speed, but they were different in degree. Extreme speed referred to the highest speed or extremely fast speed, reaching the limit of speed. And rapid speed referred to extremely fast speed, usually used in emergency situations. In terms of speed, Extreme Speed was faster than Rapid Speed. Fast speed also referred to extremely fast speed, which was similar to rapid speed. In summary, extreme speed was the fastest, and the speed of extreme speed and extreme speed was relatively faster.
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.