Could somebody from the North be part of the Kingsguard? When they make Sandor part of the Kingsguard, Barristan and others are upset because Sandor is not an anointed knight with the seven oils. They do not follow the Seven in the North and for the most part, do not get knighted and called Ser. I do not remember any Northmen being part of the Kigsguard. This is kinda odd that they would not include a major section of the Seven Kingdoms and one, with wilding raids, probably fought a lot. Also any of the Iron Born because they followed the Drowned God.
There are northmen who are knights for instance Ser Jorah Mormont.
A select few houses do follow the seven so a northmen could technically become a knight of the kingsguard but I don't know of any off hand that have been.
I do think there has been an Iron born Kingsguardman but I can't think of his name and I could be wrong.
The north and the ironborn are by far the best warriors in the realm in my opinion. Just living in those places ensure a toughness that is not the norm in other regions.