Wearing equipment that boosts willpower will eventually grant you the skill. Though, it takes SP to use the skill without wearing the equipment.
Certain clothing can also boost willpower; however, those only apply when worn.
1. Thank you, but how do I improve the number of sp in order to gain willpower ? Is it by leveling up ? (I'm actually level 80).
2. I'm asking all that because the pirate captain asks me to bring back (I already brought her and freed her) galiene on the ghost ship but a can't. When I take her with me in the team I can't reach the docks in the city desert so I thought that I could do it by possessing her.