Soul/Kakki

Kakki (活気; lit. "Lively") is the aspect of the soul that represent's your life force. This part of the spirit something that is often set in stone. It serve as a battery of sorts, anchoring your spirit to your physical body, and thus being able to live. The Kakki, however, is the part of the soul that gradually burns out as it sustains the other functions. Eventually, as it depletes, a person will reach their natural life span and die.

Overview
A person's Kakki is the inherent power source of a soul. All previous functions of an individual's spirit are solely possible due to their life force being an anchor, granting it the soul a capacity to carry out it's daily activities. It's located in close proximity to the Genki, and as such, the energy lingering within this component can potentially be used in conjunction with a person's spiritual energy.

While a person's Genki may be improved or increased, there seems to be no known methods to improve one's Kakki. Although, this isn't due to lack of trying. Scientists theorize that with a potent enough power source and technological advances, humans may one day be able to have essentially infinite life-spans.