Source of power for Martial Artists. Kei users are special since they have an organ within their bodies that produces this energy and Kei users can use it in many ways. There are different types of Kei each with its own qualities. In the Anime it is said that the "Heaven's Blade Receivers" have "monstrous" amount of Kei that makes even the strongest Kei users look weak (e.g. Layfon beat Haia and Gahard easily)
External Type Kei: Used for attack. This Kei is used in the form of a blast to affect surrounding materials.
Internal Type Kei: Used for defense and regeneration. This Kei is used to strengthen parts of the body during battle or to speed up regeneration. Layfon claim that it's bread and butter for Heaven Blade's Receivers to fight continuously for a period of days without getting tired or unfit for battle.
Psychokinetic Kei: Users of this Kei (who are also called Nen-I users) are able to communicate telepathically as well as move objects around telekinetically. The downside of this Kei is that those who are able to use it cannot use External Type Kei or Internal Type Kei because their psychokinesis is a fusion of both Internal and External Kei. A person must be born with a special physical constitution to use this Kei, while the other two can be utilized by anyone with training and a certain level of Kei