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.
In fanfics, the writer has a lot of creative freedom. Percy's crush could be a completely new character created by the author just for this story. Or it could be an existing character that has a different relationship with Percy in this fan - created narrative. For example, it could be a minor character that the author decides to develop a romantic plotline for with Percy.
One character could be a talking animal, perhaps a fox with the ability to transform into a human - like form. The fox could be a trickster, leading Gojo into all sorts of adventures and misadventures. There could also be a group of knights in shining armor, but with a dark secret. They might initially seem like the good guys but as the story unfolds, Gojo discovers their true nature. Additionally, a little girl with a unique power that she doesn't fully understand. Gojo takes her under his wing and helps her develop her power while also learning from her innocence.