Will is on of the 8 base attributes assigned to characters based on their class. Will affects a character's maximum MP and their innate ability to regain Mana.
Caster classes such as Clerics, Chanters, Sorcerers and Spiritmasters benefit more from higher will than melee classes. When a character ascends at level 10, there is an increase to the value which subsequently cannot be increased or decreased.
As with any attribute, Agility can be altered by Consumable items, such as Food, Potions, or by Spells.