Aura

Aura is the manifestation of one's soul, and can be used for a wide range of abilities. The specific abilities and their strengths differ from individual to individual and are dependent on a number of factors, such as experience, training, and innate skill.

(Information)
Aura is an ability that all those with a soul can use, at least with enough practice. A person with a strong Aura may radiate bright colors when using it. Because Aura is the manifestation of the soul, it can appear very differently depending on who is using it. The color of Aura differs from person to person.

strength of an Aura degrades with continual use. This limits the applications of Aura in combat as, with time or heavy use, a person's Aura will decay to the point where they are left weakened or even incapacitated.

Use
Aura can be used in various ways
 * Defense - Aura is mainly used for defensive purposes. While users of Aura will not be hurt as long as their Aura is up, they can still feel sensations such as pain and heat.
 * Healing -  those with a strong enough Aura can heal minor wounds.

Variations
it has been noted that identical twins basically have one aura divided between the two of them so it’s safer for them to stay together, especially in battle. their auras can only heal fully if they’re in close proximity. one twin can also transfer their aura to the other through contact if one is more injured than the other, but afterwards both will refill to the normal amount