The Wave Energy (Hadou) is the wave of life-force that flows through the human body. The size and balance of the Wave of an individual is determined at birth and it has been heavily hinted that members of the same bloodline tend to share similar Wave balance. Just like the Rings and Box Weapons, the Waves are classified according to the Seven Flames of the Sky.
Introduced during the Future Arc, the Wave Energy is an important element during the first stages of said Arc as it serves to explain the power of the Rings. A Ring will react to a Wave with the same attribute as itself and convert it into a high-densified form of energy, the Dying Will Flames.
While humans can have a number of Waves of different attributes flowing inside themselves, usually only one of these Waves actually has the capacity to ignite the Flames of a Ring. Also, cases of individuals with multiple Waves that can light up Flames - and thus can carry Rings and Boxes of different attributes - are known to exist. Still, even among those, it is rare for someone to become skilled enough to use Flames and Boxes of different attributes in battle. Incidentally, individuals that possess a Sky-attribute Wave Energy are regarded as exceptionally rare.