Spirit (スピリット) is a type of spirit strength activated only when the wielder is at a high danger, or when the user in their conscious will, sacrifice themselves, to obtain eternal strength.
Spirits were originally thought to be supernatural strength, that existed within the Gods. But after the existence of auras, they were proven to be present inside each being. But activation of spirits is not an easy task. It requires a great amount of danger to be near the wielder to allow them to activate this spiritual strength. A spirit could also be activated by sacrificing one's soul to a Lord or a Demon, but the drawback of this activation is that the wielder will suffer major harm from the negativity of the Lord or the Demon, from which the activation took place.
Spirits provide immense strength to their owners. They act as a reserve strength for them, to encounter the danger that might cause harm to them. But the risk involved in the strength of a spirit is that if it not handed properly, the strength may not work at all.