Soul

The Soul, along with the Body and Heart, is a part vital to the complete existence of any living thing. The soul literally gives the body life, it is the difference between being alive or dead. It is the force that wills the body to live, and drives it to survive.

When someone becomes a Heartless, their heart, the thing that controls the body, is released and they leave behind their body and their soul. Usually, the body and soul, having no guidance, wither away. However, if that person's heart was strong enough, then the soul will take command of the body and will it on. This being is called a Nobody. Strong Nobodies retain their human form. These Nobodies form Organization XIII, and are able to retain their form by having a strong will as a human. Therefore, it is clear that will is related and an essential part of the soul.

Unlike the heart, which controls the body, telling it what to do and giving it feelings and emotion, the soul is the energy and life the body uses to carry out the heart's commands. Essentially, the Soul is life. Therefore, it is assumed that the soul is the spirit or ghost that leaves the body upon death. The body has no energy or drive to carry out the hearts orders, so ultimately the being dies.

The heart has a connection to the Heartless while the Body has a connection to the Nobodies. However, there isn't an enemy of darkness connected to the soul yet.

In Kingdom Hearts Birth by Sleep, when Terra's body was possessed by Master Xehanort, it seemed that his soul took possession of his armor. Despite not having Terra's heart or body, the armor was able to attack and even defeat Xehanort. This could be a foreshadowing of the soul's capability in later games of the series.