Ki is a type of energy that is related to the will and the soul, but not the Resonance Soul. Every person has ki, even one without a Rez. The more ki, the more amazing things a person can do. Some abilities can be mastered only using ki. These abilities are the enhancement of speed, strength, and agility. Although in more notable individuals and masters, can allow for teleportation, future sight, and inherent wisdom.
Some Resonance Souls may be the ability to do something one could do with ki, however in this case, the Rez would more specifically be able to do this ability without the use of ki. An example would be future sight.
Some people can do this through the mastering of their ki, while others will be born with the ability and can do so without the manipulation of any ki.
Each person has only a finite number of ki. Once the ki is expended, they must rest. How fast the ki repenishes itself depends on the will of the person utilizing it.