The Soulless are those who've lost their Souls to Demons. All Demons above the age of around 70 can steal the Souls of non-Demon and mortal beings.
Those who lose their Souls are actually doing a transaction. In exchange for their Soul, which allows them to have emotion and most of their free will, they gain incredible powers that vary from person to person. This loss of emotion is a bit strange, as the person remembers the emotions and how they feel, but they cannot feel them.
There are three powers that can be gained through the transaction, depending on the power of someone's Soul.
Lower-tiered Souls offer the ability to shapeshift, and turn invisible. The person's skin also turns white, their eyes become a deep red and their blood turns black.
Middle-tiered Souls offer the ability to cast devistating max level spells in every category. Along with this, the ability to grow wings, horns and a long, spikey, whip-like tail. The person's skin turns a light red, their eyes turn black and they no longer have blood, nor do they need any.
High-tiered Souls offer the person the ability to open portals, control the minds of those around them, grow up to six wings, they grow ornate horns, and do not feel the effects of a low Mana count.
Soulless are considered outlaws, who are not allowed into cities. They are basically Demonic thralls, meaning they have no choice but to carry out the commands of Demons. However, the powerful Soulless are able to ignore the commands of Demons, due to their strong wills.