For my book, I have made a family tree which I hope to include so that readers can keep track of who is who, but what information should I include about each character?
-Their appearance so the reader be able to create an image in their mind. (A line from 1984 by George Orwell,
"Winston was dreaming of his mother.He must, he thought, have been ten or eleven years old when his mother had disappeared. She was a tall, statuesque, rather silent woman with slow movements.and magnificent fair hair"
-The top part of their personality/skills/behavior/thoughts/ideas whether all these elements or some of them, according to the story, (based on your own desire that how you'd like to do it)
-You can think of adding particular relation or behaves between the characters. If they might love , hate or jealous of things about each other, if they have unity or split,
-You can also try adding some "past" memories or "past effects" that impressing today & it's events.
That's what i could think of right now.
A small illustration of the character's face is a nice feature.
If it is related to the story their occupation or a special skill.