Aura
Narrator: Pyrrha Nikos.
Aura, according to everyone in Remnant, is considered to be an energy field created by the body. And while no one really knows where it comes from, many schools of thought believe it to come from a person's soul.
It can be unlocked, but only through one of two ways: Another aura user, or traumatic scenarios. Aura functions primarily as a damage buffer, protecting the user from harm. It is thin, and the amount of damage the user is protected from varies with the individual and depletes with active use. Should a person run out of aura, they will enter a weakened state and become vulnerable.
Aura users can easily turn their aura's on and off once they've learned about it a bit, but it passively turns on when it's first unlocked. Most people only really activate their aura in a high risk environment.
The amount and toughness of a person's aura is usually measured by two metrics: Durability during stress and duration without. Other factors include general aura amount that a person has, and how fast they can personally recover aura once it's depleted. These measurements are usually measured by durability being given a letter grade (A-Z), Time (1-43) hundred minutes. Amount is also represented by a letter grade (S being the largest and E being the smallest), and recovery speed is represented by a number scale (slowest being 120 and fastest being a 0.1) in seconds.
The amount of aura in a fighter is gauged by how many minutes of their aura can be sustained without it shutting off for recovery and without further abuse. Those minutes can also be burned by using aura for things like acrobatics and the like.
Additionally there are various forms of competition that take these factors into account in order to determine tournament brackets, such as the Crucible or the Crimson Banner. But that's a story for another time, I suppose.