Soul/Kiryoku

Dubbed as a person's will, the Kiryoku (気力; lit. "Will Power") still remains an object that garners great attention. Scholars still debate just how prominent this component of the soul is when it comes to determining the existence of free will - which serves as the basis of humanity and human existence. While the location isn't confirmed, it's said that this ethereal sector is perhaps the most difficult to reach.

Overview
Kiryoku (気力; lit. "Will Power"), refers to the aspect of the soul that governs one's will. This aspect one an individual's soul is imperative to grant free-will. Every action that is willingly done by a person is due to the soul's capacity to allow an individual to perform an act based on their own volition. The Kiryoku is noted to be the most difficult part of the soul to fully breach, as able to do so can effectively strip a person of their free will. Although exceptionally powerful entities in the multiverse are able to exude a powerful  'Killing Intent', which is known to at the very least dull a person's spirit, and make them less capable of acting even if their mind is telling them to.

An entity's Kiyroku is more susceptible to control based on their sentience. The primary manipulator of this aspect of the soul are Tamers and Yōkai Summoners, as they can often bend their conjurations to their will. However, there are a plethora of other means to manipulation the Kiryoku, although no one has seen to be able to fully gain dominion over an entity's spirit - save for those able to resurrect deceased souls, and purposely leaving this aspect out.