Will is a representation of how 'into it' you are in this fight. It starts at 100 and generally only goes up, although there are a few Spirit Commands that use it up (mostly healing abilities), and one very rare attack effect that reduces it in its target, Enervate. Will maxes out at 150 for most people. If you have the Will Breaker skill, you do not lose Will from Enervate attacks and your maximum is 170. However, most one on one fights don't go that high.
Many attacks have a minimum Will to use. Attacks that are reasonably easy to use but not used in the first rounds of a battle have 110 minimum. 'Ultimate' or 'final' attacks may require 130 or even 140. Will is never spent to use these attacks; you just can't use them before you have the required amount.
Everybody gains Will from hitting, missing, dodging, and being hit. Based on their personality, they will gain additional Will from one of those actions.