Personal Guard

This is a type of a fellowship that your knights can enter into. Professional guards are needed in many noble households as there are countless dangers lurking around in the world and you usually do not wish your leaders or heirs to succumb to them.
As the name suggest the main mission of a personal guard is to guard someone or something against absolutely anything that could potentially threaten them (including themselves) and you must be absolutely dedicated to your orders and willing to sacrifice your life for the thing or a person that you are guarding. Because what use is a guard that cannot actually guard what he was hired for, so you can be assured that you would lose your life no matter what, really. Unless you find a way how to recover what has been lost.
The highest career progression that personal guards may dream of is to make it all the way up to the royal guard and to devote their life into protecting what is the most precious in all of the land. The monarch themselves and their family and any asset that they hold dear and fight against anything that could personally threaten them or the realm.
The type of missions that your knights might expect to be undertaking have something to do with whom they decide to serve - what clan or a resource they swore to guard. Depending on that you can expect to uncover sinister plots against those you protect, investigating potential supernatural threads to their seat of power or accompanying their delegates to negotiate with neighbouring lords and ladies. Missions can be anything from dispelling curses, hunting for antidotes to poisons. Deflecting sword blows and arrows in a battle or escorting certain people to safety in disguise.
It helps if your knights have some reason beyond money to devote themselves to be guards, if money and riches is what you are after, consider taking a look at mercenaries.