Mercenary companies are para-military groups that fight by contract rather than fealty to a monarch. However, they are often formed and lead by nobility (e.g. dukes, counts). They can serve as supplements to the standing royal forces. Many are known by their commanders (e.g. Phelani, Halverics).
Phelan's company and other northern companies (from Eight Kingdoms, like Halverics) are known to be extremely honorable. In that they won't harbor criminals or traitors, torture prisoners, steal supplies from peasants, or destroy crops. They only try to kill in battle, because that's business. Southern companies (from Aarenis) have a poor reputation, to the point they can barely be called mercenaries. They are known to blackmail towns and torture prisoners depending on the group.