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.
I recommend "Peach Blossom Parting". The female protagonist in this book was also a princess, and the emperor was her brother. There was also a plot of a son-in-law and the king of an enemy country, but there was no plot of twins. However, I believe that you will like this novel as well. I hope you like this fairy's recommendation. Muah ~😗
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.