Kindness is a type of human soul trait that a human can have.
They appear as green hearts. Humans who have this soul trait usually have green eyes and green hair. This isn't always the case though, as a human can have a different hair color. If Frisk's soul becomes green, their soul won't be able to move and a blue spear is provided to block the attacks.
In terms of quantity, kindness is one of the rarest traits
This can be concluded because of the cruel human nature that humans possess. While all humans posses kindness, very few will decide to practice it and live by it with enough will power to make it their trait.
Kindness wizards, while very rare, are extremely powerful. Their defensive and healing magic is unique and very well received among their allies.
Ironically, as powerful as it is, the wizard itself is quite vulnerable when confronting an enemy. Due to their soft heart, a kindness wizard in combat, is more likely to get killed by their enemies.