Ki Energy is the subconscious and unseen life force in everything. It is an energy uniting all, whether it is visible or not. It is a important part of martial arts and can be used for healing, breaking, and defense. Animals are said to be more able to focus this energy as a result of their better instincts.
In Mini Ninjas, it also acts as a form of stamina, being the source of energy required to use Kuji magic and sprint. If Ki energy is drained by a ghost and the ghost continues, health will be lost.
'Ki energy' is similar to 'Kami', which in Shinto, a Japanese religion, is the belief there is a soul or life force in everything.