During this time of clan warfare and discord certain ninja choose to take advantage of the way the ninja world is ordered -- into nations. Whereas many ninja are simply given assignments from varying Feudal Lords and their political rivals, there are a select few who swear themselves to these Fedual Lords, becoming what is called a Ninja Guardian "Kōken'nin Ninja". These Ninja Guardians serve as elite bodyguards to Fedual Lords and have broken off all ties with their clan, instead choosing what they consider a higher authority to protect and serve - the nation itself.
While there are surely many ninja who seek to pursue a place by a Fedual Lord's side permanently, they have to reach certain requirements and are elected into the position by the Feudal Lord him/herself. They are also evaluated, often through trial, as to where their true loyalties may lie and if they truly have what it takes to renounce their clan. That said, a maximum of four Ninja Guardians per Feudal Lord has been standardized in common practice.
The typical threats that these Ninja Guardians face are assassins, their Feudal Lord's rival armies, former clansmen seeking to restore honor to their family name and rogue ninja, though there are of course many threats faced as a Ninja Guardian. Since they've renounced all ties to their clan, there's an uneasy relationship between a Ninja Guardian and the clan he/she comes from -- usually.