Oftentimes, when a soul dies tragically, and whenever a Reaper harvests that soul,
there’s some soul residue left over. Reapers can absorb this residue energy for extra use or, in this case, create a Spirit Animal.
which essentially serve as companions and familiars for the Reaper. All Reapers can create Spirit Animals, but its success varies on the power and skill of the Reaper.
Spirit Animals can be any animal, both normal, and fantastical, but what’s important is their colors.
When the energy is harvested, it may be contain something called 'Wraith Corruption' which accumilates the later a soul is harvested.
The amount of 'Wraith Energy' affects how the Spirit Animal appears, and the less it has, the more blue and bright it is.
Unlike how many think, the more or less pufified the energy is upon formation does not affect it's power, rather it's power is tied to the Reaper that formed it. The Spirit Animals of Dominions, for example, are capable of communication between themselves and the Dominons in far away distances, and the Wolves of the Dominion Matthew (Mercedes and Melchoir) have been observed to form portals whenever they bark.
However, it's been observed that Spirit Animals that are not purified are capable of tracking down Wraiths and Eidolons with ease compared to Spirit animals whose energy is mroe purified. As such, Reapers that are gatherers prefer to create them compared to spending time purifying the energy.
Nowadays, as a sort of 'Graduation Homework' Reapers training to gather Souls are required to create a Spirit Animal, with the form of their choosing.