The soul is made of three layers.
The soul, the source of power for humans and monsters. Cloaking themselves in their souls, the most brittle existence, the astral bodies Establishing themselves in power, spiritual bodies. Those who have a direct connection to this world, material bodies.
The soul is the core of every entity, there is no “being” outside of it. Any kind of consideration requires the presence of an astral body. Of course, if a being posseses nothing more than an astral body alone, the being will dissipate into thin air. So a medium to record the memories is needed, the spiritual body. However, the spiritual body preserves those kind of like virtual memory, therefore it cannot serve as a permanent medium. Hence the fleshly body is needed.
So for those with tempered souls, should they even sustain brain damage, recovery of certain memories becomes possible. And, there are monsters who live with only a soul. That kind of being, however, is inferior to the rest. But, even if it is but a soul, should it obtain some level of intelligence, it will be recognized as a monster.
These are what the world knows as the four True Dragons, the highest form of existance. But, even this special race requires a soul.