A good Ki build for the story in Xenoverse 1 could focus on putting most of your points into Ki Supers and Ki Blast Supers. This way, you can deal a lot of damage from a distance, which is useful in many story battles where you might face multiple enemies at once.
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.