The main character in 'I am the Sorcer King novel' is the Sorcer King, of course. There could be other significant characters like his loyal servants or powerful adversaries. It's common for a novel with a powerful protagonist like a Sorcer King to have characters who either support or challenge him. For example, there might be a young apprentice who looks up to him or an evil wizard who wants to overthrow him.
The Jujutsu Kaisen manga is indeed ending. It's been a great run, but all good things must come to an end. The final chapters are bringing closure to the characters and story arcs.