Hikari ochiru is essentially a crossover series between several manga, game and anime franchises, such as Naruto, Mai Hime, Kingdom Hearts and Final Fantasy. The story focus on Andre and his friends as they journey out to save the world and the universe from a horrifying fate, the story follows behind the plot of Kingdom Hearts seeing as how the two connect.
The element world is a fictional world that is opposite the real world. It is where Andre and his friends discover they are connected to the world and that they are the inheritance of people known as "fighters" which are a group of people who protected the Earth for many years from the shadows of darkness. he element world is also deeply connected with the real world such as anything harshly damaged in the element world impacts the real one an example of this is when Andre for a last resort summons all his power to defeat Chi sending her into the world's core but due to the impact the world is damaged and begins to split this causes Earth to split along with the Element.
Ki (気) is the force energy that holds the power of one of the world's major elements this is also something that fighters and only fighters can use. This ki comes in the style of Water, Wind, Fire, Earth, Light and Darkness and a fighter who holds that ki can use a energy based attack off of that element e.g: Andre is an inheritance of the Sky and one of his ki moves is the Sora dageki literally meaning Sky strike.
The term "Ki" was first introduced by Mutaito in a filler episode of Dragon Ball where Goku travels back in time and learns about Ki. Although it is unknown who originally discovered this inner energy in the Dragon Ball timeline, the first usage of it is seen when Master Roshi performed an energy blast called the Kamehameha in order to extinguish the fire on Fire Mountain. By drawing his ki energy into the palms of his hands, Roshi was able to expel an explosive mass of visible ki energy. This was the birth of Dragon Ball energy attacks.