According to the Secret Ansem Report 4, written by Ansem the Wise during his time imprisoned in the realm of the in-between, the soul is a vital part of a complete being, along with the Body and Heart. Supposedly, the soul gives the body life, marking the difference between a living and non-living entity. It is the force that wills the body to live and drives it to survive.
According to the Ansem Report 13, when someone becomes a Heartless, their heart, which controls the body, is released, leaving behind the body and soul. Usually, the body and soul will wither away. However, if a being's will is strong enough, the soul will take command of the body, creating a new being that lies neither in the light nor the darkness. This being is called a Nobody.Exceptionally strong Nobodies, such as the members of Organization XIII, retain their human form. Therefore, it is clear that will is an essential part of the soul. This is further supported both within Secret Ansem Report 10 and also when Terra-Xehanort first faces the Lingering Will and refers to the heart, body, and mind (as opposed to soul).
It can be summarized that the heart provides moral guidance and emotion, whilst the soul is a sort of spiritual life-force the body requires to carry out the heart's commands. Therefore, it is assumed that the soul leaves the body upon death. If the soul leaves the body, it has no energy or drive to carry out the heart's orders, causing the being to die.
In Kingdom Hearts 3D: Dream Drop Distance its is revealed when a Heartless and Nobody are defeated the Soul, Heart and Body reunite. The memories of the Nobody appear to be imprinted on the soul, allowing the reformed being to recall the missing time during their separation.
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