(last updated: 12/28/17)
Aura is a Defensive stat, and it functions to aid you in resisting Chi attacks. A high Aura score will reduce the damage taken from any Chi attacks that hit you, and also helps in successfully Blocking such an attack. It also helps you succeed at Just Defended attempts against a Chi attack, assuming you have the Just Defended ability.
Aura is, generally, something you can only train at to improve; there are few natural physical characterstics that modify this. By and large, those who train frequently at wielding Chi energy understand it well, and therefore also have correspondingly high Aura scores.
Those who don't train frequently with Chi energy may well still rate at least the average score in this stat for a character of their Ratio though, but it is nonetheless possible to have a below-average score in this stat.