Will is the Defense stat that measures your resistance to Psi, that is, Psycho Power attacks. A higher Will score will cause you to take less damage from a Psi attack, and also makes you more likely to successfully Block or Just Defend a Psi attack, though once again you'll need the Just Defended ability if you want to attempt to JD.
Will is an oddity in some regard, as its score is based on two major aspects. First, how familiar the character is with Psycho Power plays a role; most characters don't even know what Psycho Power really is, let alone encounter it frequently enough to really practice major defenses against it.
A character's personality also helps, to a small degree. Someone who is extremely confident may have a slight edge in Will.
Will is the only stat where the Average Score is ten points lower than usual. Thus, while the average stat for a Ratio 2 character is 20-29, the average Will stat will be 10 lower, 10-19.