Something I've wondered for a while now... What do Friends and Rivals exactly do? I reckon that you get some sort of diplomatic bonus when dealing with count/duke/king friends, but rivals? It's harder to succesfully perform diplomatic actions with rivals?
Friends can give you money and/or troops during war (if they are rulers). Friends in advisor positions can give you claims, money etc.
Friends in court and vassals gain loyalty bonus
Rival rulers I am not sure about, but I think maybe some events against each other are possible.
Rivals in advisor positions is NOT desirable. Stewards will steal your money, spymasters will try and kill you, chancellors will loose you prestige etc.
Rivals in court and vassals gets a loyalty deduction.