Well, 'an arc to the past rwby fanfiction' could be about a lot of things. It might center around a character who discovers a way to journey into the past of Remnant (the RWBY world). Maybe it's about uncovering secrets from long ago that have an impact on the present situation in the story. For example, it could be about a character finding out the true origin of the Grimm and how that knowledge changes their perspective on fighting them.
I don't know for sure as I haven't read it. But in RWBY, it could be Ruby, Weiss, Blake, or Yang. Maybe it's about one of them going back in time and becoming a main focus during this arc to the past.
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.
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.