Ki is a power for a Martial Artist and the Willpower of the user. It is used by martial Artist Fighters.
Every person who trains in Martial Artist is able to use ki in fights. The users of this technique can drastically increase their attack and defense power to inhuman levels with proper training and control. The user of this technique must know the type of Ki that they have, either Sei or Dou Ki. Some fighters can even use both types at the same time, achieving the ability use what's called Dual Ki. According to Hayato Furinji, there are various levels of Ki useage, but the three main stages of use are:
- 気の発動 (Ki no Hatsudō), known as the "Invoking" of Ki: which allows the user to channel their Ki inward into their techniques and attacks. Most disciples of all levels, Low, Middle, and High are capable of reaching this stage.
- 気の開放 (Ki no Kaihō), known as the "Release" of Ki: which allows the user to release their Ki outward from their bodies and use it in various ways against their opponents. The only disciple leveled fighters capable of reaching this stage are High Class Disciples. The Experts and all of the Master Class levels have achieved practically endless uses of this stage of Ki control.
- 気の掌握 (Ki no Shōaku), known as the "Holding" of Ki: which allows the user to not only release but freely control their Ki in and out of their bodies, even able to pull in and redirect the Ki of other people. Masters of the Grand Master level fighters have managed to perform this stage of Ki control, but the only ones who have mastered this stage completely are the fighters who have come to be known as Legendary Masters. Despite it's difficulty, Kenichi has proven himself as the only known High Class Disciple to demonstrate this ability in combat.