Faie, in the universe of To Your Eternity, refers to the "soul" or "spirit" of a living thing.
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 "faie," which may be transferred to a new vessel in the process of reincarnation.
The loss of faie occurs when a living being loses it's capability to feel and becomes "empty." Faie is the energy required to move the vessel. If the vessel is broken, the faie will be released. As such, faie represents the will of doing what you want or going where you please.