It starts at a specific point in the manga, but I can't recall exactly without looking it up. You might want to check some manga forums or dedicated fan sites for the precise chapter.
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, popularity is relative. In some fanfiction communities, they might be quite popular because they offer a different take on the well - known Death Battle concept. People who are interested in exploring the characters' relationships in a more...shall we say, 'intense' way might be drawn to these. But in the broader context of all fanfiction, they are probably not as well - known or widely read as other types.