Fye

Fye, in the universe of To Your Eternity, refers to the "soul" or "spirit" of a living being from humans to animals.

It is a common belief that, in order for an animal to move there must be both a bodily vessel and a means to fill it, denoted "fye," which may be transferred to a new vessel in the process of reincarnation.

The loss of fye occurs when a living being loses it's capability to feel and becomes "empty." Fye is the spiritual energy required to move the vessel. If the vessel is broken, the fye will be released. As such, fye represents the will of doing what you want or going where you please, even choosing to reborn/reincarnate in the form of another; namely Pioran and Hayase.

Trivia

 * In the anime and manga of InuYasha and its eighteen/fourteen-year long spinoff Yashahime: Princess Half-Demon, it is called haku, whereas the mind is called kon.