Nature is a vital part of everyone's energy. Energy can either be considered light, or dark. However, a person's energy nature has nothing to do with their personality or motives, so kind-hearted people can be born with a dark energy, and sadistic, heartless people can be born with a light energy.
Light energies focus on creation and power. They have less destructive power, but greater range and variety. It can enhance the user's physical strength, and it makes it easier for them to learn new techniques, although the techniques may not be as powerful.
Dark energies focus on destruction and speed. They have more destructive power, but less range and variety. It can enhance the user's mental strength.