Every character has a base "Nerves" and "Capacity" that are derived from their attribute modifiers. For every skill level a character recieves in "Willpower," the character's Nervesand Load increase correspondingly. Furthermore, once a single level of Willpowerhas been purchased, characters can specialize in Nerves or Load if they so choose.
Nerves = character's ability to withstand attacks on their psyche (both magical and non-magical), MP values and regeneration, and mental pain tolerance
Capacity = Mental capacity for performing instensley cognitive tasks.
(see Character Creation for further info)