As mentioned above, your kingdom’s development in various areas is quantified by your kingdom stats, and each of these kingdom stats can have an Advisor assigned to oversee the maintenance and improvement of these vital state functions. The primary function of Advisors - and at the end of the day, of all aspects of kingdom development - is to boost your kingdom stats, and they do this largely through the completion of Event Cards, which will be covered in greater detail below.
While many Advisors (especially initially) are your adventuring companions, others can be found and recruited from outside the ranks of your fellow party members. Whether they’re willing to join you often depends on what choices you make during specific quests, especially if that choice possibly involves a violent altercation with said Advisor candidate! Generally speaking, sparing NPCs and not being overly harsh in your interactions can lead to you having a somewhat fuller throne room in the long run. There are also instances where your Leaders can die in action, depending on the choices you make. Possible Advisors, how to recruit them and how to keep them alive afterwards will be explained throughout the walkthrough.
Advisors aren’t mindless automatons - they have their own traits and beliefs, and will act accordingly. Not only will the individuals you promote flavor your reign, but which Advisors you assign to handle events represents the nature of your response. Say, for example, that there’s civil unrest in your lands over some matter or another. If you send the General to deal with it, that indicates a military response - harsh repression of troublemakers who may or may not have legitimate grievances. Should you send your Regent, the issue is understood to have been addressed more peacefully, through diplomacy. In practical terms, this means the Advisor you employ determines the statistical outcome of the event, usually by gaining your kingdom stats in the field that Advisor represents.
As you complete events and raise your kingdom stats you’ll reach thresholds (every twenty points in a kingdom stat) at which said stat can rank up. The benefits this bestows has also been covered above, but there are intricacies to the process that don’t directly affect kingdom stats, and that’s what this section is for.
When a kingdom stat ranks up, the Advisor responsible for the kingdom stat will request an audience with you, obligating you to meet with them in your throne room. During this meeting they’ll present you with a problem related to the area they represent - the Regent may need your help resolving a civil dispute, or the General may require your input in enlistment practices, as examples.
Your input here will help shape the nature of your kingdom, but your Advisors may have their own opinions on the matter. While you can usually give them free rein to do what they wish (which will almost always appease them - they’re not inclined to act against their own inclinations voluntarily, after all) you can also make choices that please or displease them. If you consistently displease your Leader with these choices, they’ll quit, forcing you to find another, hopefully more like-minded individual to fill the role. To avoid messy political spats and to ensure your chosen policies are implemented with as little friction as possible, it’d be wise to pick Advisors whom you find agreeable. A full list of these rank up events and their Leader-influencing choices can be found in the Kingdom Rank Ups section of the guide.
If you have created NPC companions of your own by hiring Mercenaries from Anoriel Eight Eyes (found initially at Oleg's Trading Post and then later in the Tavern in your Capital (once you build one there), do NOT use them as Advisors. They have a built in negative value in resolving Events (-4 or -5, can't recall which). Although this can be improved by boosting the relevant Ability score that relates to the Advisor role you assign them, they will never be as good as the NPC companions that are available by default (Harrim, Amiri, Valerie, etc). Obviously you can still use such NPCs on adventures and just use the prebuilt ones as Advisors. It is still advisable to give your advisors items to boost their Advisor stats. The relevant Advisor stats are as follows: Military = Str, Regent = Cha, High Priest = Wis, Treasurer = Int, Councillor = Wis, Grand Diplomat = Cha, Warden = Con, Magister = Int, Curator = Cha, Minister = Dex.