Souls

The Soul is an animus of significant enough size to be sapient and to inhabit and control a complex body like that of a human. Most animus take a form in order to experience the world, though some remain intangible, and the most powerful can take and leave forms at will. The animus is a construct of pure aether, and should be considered the source of a human mind and lifeforce.
The Soul is fully aware on its own, but while contained in a mortal body it will rarely remember details of its former vessels lives. The very oldest souls may sometimes recall snippets.
The size of an animus dictates the ability to perform magic; the soul has the ability to expend some of its aether in order to manipulate reality, but small souls do not have the excess aether available to do so, and larger souls have more excess and so can perform more magic.
Most animus are too small to manipulate magic; a critical mass is required in order for an animus to sustain itself, else it would dissipate. If the soul falls below the necessary aether, it would try to continue its existence by draining the closest matter, the vessel body, of aether; leading to death. In extreme cases, the soul might dissipate if it fails to gather enough aether to save itself. In cases of normal magic use the soul will then replenish the used aether from the free floating environment, until it reaches its former size. Souls grow larger over time, and therefore more powerful in the use of magic.
Souls are most strongly attracted to vessels related to their former vessels; for this reason, while magic is not necessarily ‘genetic’, souls will most times be reincarnated into a descendant of their last vessel. Therefore the ability to do magic will follow. In cases where a family goes extinct, or does not have enough descendants for all the souls of ancestors, the soul will find a new vessel elsewhere.

Soul Templates

Every animus possesses a so-called soul template, a blueprint of the correct nature of its body. Some souls possess more than one, in essence allowing shapeshifting.
It is impossible to alter the body significantly away from its template, but the template is flexible to a certain degree; magic has been successfully used to change hair colour and eye colour, and it cannot stop material damage or alteration to the vessel - ranging from tattoos to dismemberment. However it can be used by healers to restore the body to its former state with enough aether, supplying a shape of what should be.
No attempt to alter a template intentionally has been successful but the soul itself can alter its template instinctively. These changes are unintentional and based on images in the vessel’s subconscious, happening only when the soul is gathering magic to quickly and must expend it or risk the animus’ stability.
It is well known that the animus will prioritise a vessel related to former vessels. However, the issue arises of “chicken or the egg”; soul or genetics. The animus dictates the template of a body, but simultaneously genetics dictate the inherited appearance. It has been theorised that the soul template is established when the soul enters the body, and not before. Because of this, the soul template can be flawed and so congenital defects are often impossible to fix with magic.

Manifestation

The soul is invisible to most creatures, except perhaps to the most powerful spirits.