It could be his dog. Dogs are often considered man's best friend, and in a story about a boy, a dog would be a very likely candidate. The dog could be loyal, always by the boy's side, and they could go on all sorts of adventures together.
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.
It's also possible that the girl's best friend is an older person, like an aunt or a grandmother. They could have a special bond where the older person gives the girl advice and shares stories from her own life, and in return, the girl brings youth and energy into the older person's life.
One great Gojo fanfic is 'Gojo's Ascent'. It delves deep into his character, exploring his powers in unique ways and his relationships with other characters. The story has a great plot that keeps you hooked from start to finish.
Li Qin and Zhao Liying had the best relationship. The two of them had worked together many times, including in 'Dream of the Red Chamber' and 'Run! Brothers " and variety shows. They worked together frequently and had a lot of time and opportunities to interact. As a result, they had a deeper understanding of each other and became good friends. In addition, the interaction and tacit understanding between the two were also appreciated by the audience. The other search results didn't mention Li Qin's best relationship with anyone else.