Listed below are some of the most common opinion modifiers, I have tried to interpret them from the game files as closely as I could:

Negative opinion of your vassals will lead to getting less taxes and less levies and all of the above it gets worse. Negative opinions of you can lead as far as wars, rebellions, abandonment, attempted assasinations, plots, and in general a character with negative opinion of you will try to get rid of you any way possible. It is usually imperative to keep your vassals or your leige happy. The better the opinion of you is, the better benefits you will receive from a character, ranging from diplomatic to financial. Opinion is a core element of the gameplay and manipulating it is the path to success. Opinion is represented by a number, red being bad, green being good, from 0 to 100. Your relatives, vassals, barons, bishops, as well as all foreign characters have opinion of you, the main character. In Crusader Kings II each character has an opinion of one another, negative or positive.

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