(last updated: 12/28/17)
The will, or drive of a person to battle is an ephemeral trait, one that does not easily translate to concrete fighting terms. While a person's Aura is a measure of their ability to control chi flowing through their body, Will is itself a measure of a person being able to control their own psyche and soul. Zen masters have notoriously high Will, and psychics naturally have higher Will due to the fact that they have already learned to control their body's energy.
Will regulates the sort of damage one takes from psionic attacks--psychic assaults simply do not have as great an effect on someone of a higher Willpower than other characters. However Will can be one of the hardest competencies to quantify--part of it is training, and part of it is psychology, meaning that a particularly willful individual may have high Will just the same as a psychic master who has trained against psychic attack all their lives--one does not strictly need to be 'aware' of psychic energy to be resistant to it.
At the same time, true Will competency is rare in the world; while fighters at higher levels will naturally have better Will due to the fact that their styles and thus their minds are more well developed, some of the strongest fighters might have emotional or spiritual weaknesses that makes them susceptible to a psychic assault.
Will is associated with no direct physical feats, unlike other competencies. However, high Will is typically a statement of a fighter's mental resilience. A character with high Will is capable of swaying others to their ways of thinking more easily, and at the highest levels of Will might actually represent a compartmentalized mind, or a guardian personality watching over and protecting a fragile, weaker one.