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 idiom of "gossip in alleys" referred to the discussions of the people in the streets and alleys. This idiom could be used as subject, object, or attribute, and it mostly referred to public opinion. Its source was Song Zeng Minxing's "Solitary Awakening Miscellany.":"From the laws of the imperial court to the miscellaneous families in the barnyard, there is nothing that is not recorded." It was a street talk.