All Heroes, Monsters, and Cards are either Neutral or one of the 5 elements.
As illustrated in the picture, each element has a weakness. When a fighter or shield is attacked by it's weakness the attack does 50%? more damage. Fighters (and Fighters with elemental shields?) are immune to their own element's status effect but still take 100% damage.
The element used by Monty, Fergus and Red. Battle Cards of this element can inflict the Burn status effect.