It's a bit tricky to pinpoint exactly. The anime's ending doesn't have a direct one-to-one match in the manga. It sort of stops at a certain point, but the manga continues the story further.
The 'My Love Story with Yamada-kun' anime has indeed finished. It might leave you wanting more, but you can always relive the moments by revisiting the episodes or discussing it with fellow fans.
Overall, the Yamada-kun anime does justice to the manga. Some scenes might be presented differently for the sake of animation, but the core remains the same. There are also added details that enhance the viewing experience without straying far from the original.
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.