A heartless is a creature that is created when you lose your heart. But some light will still stay in you, meaning that your heart, although is missing, you can recover it when someone shows affection to you or when you are pure enough. A nobody is born the same time as a heartless. Death of a heartless and nobody leads to resurrection. It happened to Sora in Hollow Bastion. He got his heart taken off, to save Kairi (his childhood friend) and became a Heartless but still was able to control himself thanks to his pureness. He came back to his human body once Kairi held him in her arms showing feelings.
There are several different types of heartless, and they can be controlled by those who have hearts. As Maleficent explains in Kingdom Hearts, those with hearts who control heartless are still potential victims of their heart-stealing abilities. In this respect, the master doesn't really have full control over them. However, those with no heart to begin with have no vulnerability to the heartless and can remain to command them unharmed.