Folks can be categorized by the element of their attacks. Generally folks are weak to opposing elements, for example: Fire is weak to Ice, but each one has individual strengths and weaknesses. The folks with the elements Water, Wind, Thunder, Charm, Bond & Sleep can only be used by Ellen and Earth, Fire & Ice are exsclusive to Keats.
Memory Stones are made up of only seven different elements. These stones are weak to attacks from the same element, for example, Fire is weak to Fire.