In addition to your main characters, you can also temporarily play bandits, raiders, fanatics and others like them. These characters are shunned by noble society and will never be accepted into a realm or hierarchy. They represent the chaotic nature of the world, leaders of peasant revolutions, bandits hiding in the vast forests or mountains and other uncontrollable forces.
In addition to some chaos and confusion, the main purpose of bandit characters is to give you an option to play the game in a more reckless, carefree way. Plunder at will, attack for fun, be unpredictable and aggressive - things you may not always want to do with your main characters.
So in a way, bandits add a kind of "NPC" enemies to the game while keeping with the core design principle of having everything in the game in the hands of actual human players.
As outlaws, bandit characters cannot partake in any part of politics. They cannot swear oaths or be members of a realm. If they get control of a settlement in any way, that settlement will leave its realm. They cannot be or have successors and thus cannot inherit anything. They cannot hold positions.
Bandits also lack the authority of nobility, and the soldiers who follow them do so for some personal, local purpose (which the player of the bandit character should define through roleplaying). They will thus refuse to leave the vicinity of their home. A bandit moving too far away from the home regions of his soldiers will face mutiny and desertions.
There are a few other restrictions to streamline gameplay and ensure that bandit characters are used for their intended purpose.
Upon death, a new bandit character is spawned into the game world, so that the number of bandits is more or less static, varying with player count in the game.
You can then take control of a new bandit character if you want, or leave it to someone else.