What do the percentages beside the elements on the character and monster stat screens mean?
These show you how well-protected you and your foes are against the various elements. 100% means no extra protection. The full damage of an attack gets through. Above 100% means you take more than normal damage from the element. Less than 100% means you receive less damage from that element. Negative percentages mean that attacks of the corresponding element actually heal you or the monster.