Willpower increases the chance of applying and resisting states. Each point of willpower will increase the original chance to apply a state by 1%. The willpower of the character receiving the state will be subtracted from the character that attempts to cause the state. If the result is positive, the chances to apply the State will be increased. Otherwise, it will be reduced.
Level 100 Rocknoceros's 10% stun chance with 20 willpower = 12% chance to stun Level 100 Rocknoceros with 10 willpower and opponent has 50 willpower = 6% chance to stun
Original Stun Chance *(1+(Willpower-Opponent's Willpower /100))