Based on the fact that the dead king kept the King of Callow in his service he seems to be a powerful name. Based on the name receiving support from each level of society the main power of the name is to call upon a whole nation to unite under the will of one individual. Desertion and infighting will be nonexistent for the army-backed by the good king.
As the "Good" king he embodies the will of the people themselves. He perfectly supports the masses of individuals both in court as well as the common people. In order to attain this title, he must be both benevolent as well as a staunch nationalist who supports his people no matter the cause. The Fairfax background of countless noble individuals also helps to support this name as the actions of ancestors can give weight to the current ruler if they are not good but are perceived as good.
The good king can command others with great efficiency. While Fairfax is seen holding a blade the true value of the name is in its abilities to rally others.