Ki is the energy utilized for “special abilities” and/or Resonance Souls.
Every person, including those without Resonance Souls, have ki. Through training, they can use their ki to preform special acts such as super speed, future sight, heightened senses and/or agility, as well as many other things.
Specialized Ki training can result in abilities that rival Resonance Souls, such as the formation of doppelgangers or teleportation.
People can be born with a Resonance Soul that is capable of doing things Ki Masters can do, but in this case, the specialty with the Resonance Soul is that the ability takes very little Ki to use in comparison to someone who has had to train for it.
People have a limited amount of Ki in their bodies. Once they expunge this amount, they must rest in order to replenish their inner supply. If someone with a Resonance Soul runs out of ki, they cannot use their Resonance Soul until a relevant supply has been replenished.