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.
The Black Flash is often associated with the Speed Force in the DC Comics universe. It is a sort of death manifestation for speedsters. Essentially, when a speedster's connection to the Speed Force is severed in a certain way, often due to overusing their speed or some major disruption in their relationship with the Speed Force, the Black Flash may appear. It comes to claim the speedster's life as if it's a reaper specifically for those with super - speed.
Indeed, Black Flash was the one to defeat Savitar. The battle was full of action and unexpected twists, but in the end, Black Flash emerged victorious.