The Kingsguard is a recruitable companion in AWoIaF.
Members of the Kingsguard cannot become lord of a fief. They do not have anyone they like or dislike and are not upset by any action.
The Kingsguard is a recruitable companion upon freeing them from captivity. They have no cost to recruit and only need to be asked and for them to accept.
When freeing a member of the Kingsguard, you'll be given three choices: Let him free, ask him to join me, capture him. If you asked for them to join you and they accept, they'll become a companion. If you choose to free them or they decline after being offered to join you, they will no longer spawn and you will not get further chances. If you capture them, they'll behave like a normal unit prisoner and you can attempt further persuasion to join you indefinitely. This makes capturing them a safer route to making them join you, but will take a much longer time to do so. (As of 3.0)
Members of the Kingsguard
There are total of 7 members of the kings guard.
They start at level 40 with very high stats, making them one of the most elite companions. They also all start out with a set of heavy kingsguard armor, helmet boots and a sword (no glove).