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.
There were two reasons why Ishida Sansei refused to drink the water. First of all, some people thought that he refused to drink water because persimmons were prone to phlegm. He did not want to die with unclean things, and persimmons were not good for the body. Secondly, Ishida Sansei believed that ambitious people should cherish their bodies and be responsible for themselves even when they were on the verge of death. He refused to drink water to maintain his self-discipline and cherish himself. These two explanations reflected Ishida Mitsunari's characteristics and self-cultivation as an ambitious person.
Well, a Nishimiya x Ishida fanfiction often explores the relationship between these two characters. It could be about their friendship deepening into something more, like a romantic relationship. The fanfiction might focus on their interactions, how they understand each other's feelings, and the development of their bond over time.
I'm pretty sure Ishida doesn't have a niece in the manga. Usually, such significant family relations would be clearly shown or at least hinted at, but there's been no indication of this.