You can approach this from several angles.
There is the celebrity angle, where these NPCs are active and involved in the world, and as members of the top of their fields they are famous, wealthy, and very powerful. Example: Arthur, Merlin, Robin Hood, the Knights of the Round Table and the Merry Men in a contemporary setting (aka they are alive)
A second angle is the historic approach, the NPCs are no longer alive, or around, but their names resonate through their professions, and the things they did are passed down through training an tradition. This would work well for genre fantasy, as you can throw names in, and let the details fill themselves in. Details can be drawn from specific settings, such as the characters in Lord of the Rings who date back to the Silmarillion, etc.
The third angle is the mythic approach, Hercules was the greatest fighter, but he was half god. The greatest members of each class are partial god, or gods in disguise, or were mortals who offended the gods and were punished for it.