They embody the virtues of wisdom, strength, courage, immovable spirit, and unwavering loyalty.

Although they have the visage of fierce warriors, they are not seen as aggressive in nature. Quite the opposite is true. The Nio are determined protectors but they are devoid of rage and cruelty, for anger is not only considered to be a weakness, but also one of the warrior's defilements.

One of the Nio is depicted with his mouth open. The other with his mouth closed. Each mouth shape represents the sound of proper abdominal breathing: "ah" and "un". But they are other meanings to this depiction. The Nio with the mouth open brings in what is good to the temple or the dojo. The Nio with the mouth closed, wards off evil from them. The mouth open also represents teaching by words (instruction). The mouth closed represents teaching by actions (example). The mouth relates to the acceptance of the teachings (willingness to be taught), the mouth closed represents acceptance of the teachings (application of the knowledge.)

There are actually three stages in abdominal breathing: Inhaling through the open mouth as if pronouncing the sound "ah" and breathing into the lower part of your lungs. The second stage is to hold one's breath as if pronouncing "un" and concentrate the energy on the tandem. In the third stage, one exhales through the nose as if pronouncing "uun." Many have wondered why there is no depiction of a Nio in the middle stage, that which is called "shisei musoku" (which means "wholeheartedness stops one's breath".) But the teachers explain that the middle stage is a transition between life and death, thought and no thought. At that point there is no actual existence but the potential of one, which will be defined by the consequence of an action, or a choice. For a samurai, this moment often came precisely at the time when he was holding a sword on his hands and facing an opponent. If he was weak and drew in a breath at a wrong time, his mind would waver and he would become vulnerable, thus creating an instant of fundamental accountability.

The samurai sought the power and virtues of the Nio. They often invoked them and tried to draw upon themselves the nature of the guardians. The Nio did not represent perfection, however. They were seen as ascetic followers of the path. Their virtues were those of valour and steadfast determination to fulfill their role at any cost, regardless of circumstances, ever faithful and vigilant.

The leading members of a dojo, both designated inheritors and senior students embraced the role of the Nio, as protectors of the ryu, the dojo and of the teacher.

By the advent of the Tokugawa era, which began in 1603, the Nio were no longer mere statues of distant deities, but the representation of the ideal warrior, the ideal budoka, strong, selfless, dedicated and a zealous guardian of "the way" and places where it was taught.

Today, that perception has not changed. In a traditional dojo, the sempai, the senior students, are still considered to be the dojo's Nio, evoking, if not invoking, the virtues of the Kongorikishi.