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 could be a lot of things. Maybe it's the backstory of a character. For example, their family history, how they grew up, and what events shaped them into who they are.
"Deception" was the wrong word. The correct word should be "tireless in teaching". This idiom means to be willing to teach others without feeling tired. It originated from the Analects of Confucius. Confucius emphasized the importance of never getting tired of learning and teaching. This idiom is used to describe the very noble qualities of a teacher or tutor.
While waiting for the anime, you can also click on the link below to read the classic original work of " Full-time Expert "!