A character bible is not just for characters but can also be used to keep track of historical events, locations and items.
In the “Lord of the Rings” series some data that would need to be kept track of was Middle Earth itself, the geography, weather etc. The ring and its power and history could also have its own bible.
For my “Character Bible” I have a brief history of the world, legends, religious belief of societies, wars and disputes between clans, mythology, prophecies and anything else that my character or the reader will discover throughout the book.
When I’m keeping track of a specific character I usually draw or find a picture of someone who looks like how I imagine they would be and paste it into my CB.
Other common things I add are:
Physical characteristics: hair color, race, body type, scars, tattoos, human vs. non-human)
Natural reactions when they are nervous, mad, in love, happy, scared.
Destinies, missions or quests
Significant trinkets (a sword, ring or necklace)
History: past, parents, trauma, discoveries.
What motivates him or her?
Goals for the future
Beliefs about the world
Beliefs about themselves
What kind of person would this character fall in love with?
What makes them angry?
These are just a few ideas.
Get to know your characters.
I spend about two weeks getting to know my characters before I start writing. The better you know your characters the more they will help you write the story. Create a character bible. Figure out their strengths, weaknesses, past history, nervous habits, favorite foods, their dreams and hopes, character flaws, and special skills. Get to know them so well that if you sat down with them in a restaurant you would know what they would be wearing, what they would order, and how they would tip the waitress. Also figure out the geography you are working with and any significant items, like magic swords or priceless family heirlooms. Objects can have their own history, weaknesses, or special powers.
The more you get to know your character the easier it will be to write your story and you will make fewer character mistakes.