Soul

A soul s the immortal essence of a person. Ideally, it re-enters the reincarnation cycle upon the death of it's physical shell.

Characteristics

A souls is highly malleable, but embodies a few core qualities that set it apart from the rest. For example, while Mizar's personality might fluctuate wildly from reincarnation to reincarnation due to the circumstances, whomever bears it will almost always be full of life, colour, and energy.

A soul are incredibly valuable, and provides a huge burst of power to a demon that consumes one. There is little else that holds such power when it comes to making a deal. In many extreme life or death cases, a soul is all that can cut it.

However, consuption isn't the only practical use for a soul. Simply owning one's soul for their own can reduce the mortal in question to putty in one's hand. No matter how many times the original owner is reborn, the contract is binding. Therefor, selling one's soul is incredible serious business. Making use of it is optional on the demon's part, of course, but it's incredibly rare for them to pass up the opportunity.

When a demon owns a mortal's soul, they become connected. Beyond just having the ability to control the mortal, the demon can often sense strong emotions such as distress across the link. It's also much easier for the mortal to call the demon's attention without the aid of a proper summoning.

Reincarnation

Before being reincarnated, souls enter an 'in between' phase. In this state, they are a vague mix of all they previous past lives. Ordinarily, they feel a sense of peace and love, being intently connected to all who ever might have cared about them in a way that would never have been possible in the world of the living. In this state, it is possible for a being of higer power (such as Alcor) separate the soul from the others, and draw forth the persona of a specific life, bringing it to the forefront- though it takes a great deal of energy to do so.

A soul can be barred from the reincarnation cycle if it is either held captive by some powerful forced or devoured by a demon. In the latter case, it will re-enter the cycle eventually, but not until after a period of time lasting up to at least a few centuries.

It is very possibe for a human soul to be reincarnated as a different animal. However, certain souls, have particular instinctual 'preferences' when it comes to whom they will be reincarnated as, such as gender (more often than not, Mizar is reincarnated female).