Increasing a creature's element will both increase its offensive capabilities when utilizing said element and defensive ability when attacked by that element.
For example: If an Efreet has 30% Fire and an Marid has -30% Fire the Efreet will receive a 60% boost to its damage against Marid IF the following is true:
If the Efreet uses the skill "Dragon Fire", his Fire affinity, which is 30%, will be added to the skill's Fire affinity, which is 10%, thus giving the skill a 40% Fire boost.
|Complete Elements List|
|Increasing Darkness will decrease Light and vice versa.|
|Increasing Fire will decrease Water and vice versa.|
|Increasing Earth will decrease Wind and vice versa.|
The vast majority of equipment will increase one specific element's power and at the same time decrease the opposing element's power, functioning much like a double-edged sword.
You can increase every element a creature has by increasing MGC instead, since it raises all elemental affinities equally, but slowly. For example, both Fire and Water will be increased by MGC.