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.
Well, dogs don't really understand the concept of stories about girls in the way we do. They may react to the tone of voice or the excitement in the storyteller's voice, but not the content specifically about girls.
There might be a reaction fanfic where they react to a new villain in their world. They would have to figure out how to deal with this threat, and their reactions could range from shock to determination. For example, Twilight Sparkle might immediately start researching about the villain's powers while Rainbow Dash is ready to take on the challenge head - on.
Twilight Sparkle is often a popular character. Her intelligent and curious nature makes for great reactions in these fanfics. She's likely to analyze situations deeply and come up with plans.