Genso no Seishin
Elemental Spirits (元素の 精神, Genso no Seishin) are magical beings residing in the Celestial Spirit World. Some Mages, called Celestial Spirit Mages due to their specialization in Celestial Spirit Magic, are able to summon Elemental Spirits by opening their Gates via the use of Elemental Spirit Gate Keys.
While the Silver and Gold Keys call the Celestial Spirits who are representative of the stars in space, the Elemental Keys represent the elements in nature. They live in a special part of the Celestial Spirit World. While they are a separate race, they have agreed to (for the most part) abide by the Spirit King's rules, such as the contracting and killing rule. Their keys can be found in their natural environment. They usually take a juvenile appearance, most commonly a boy or a girl, but in some cases, they take the appearance of an animal commonly found in their respective environment (i.e. higher rank Earth Spirit takes the form of a stag).
Each type of the Elemental Spirits has their leader, the strongest one among them. It is said that their keys are the hardest to find and that their strength rivals the one of the Golden Keys themselves. They often refuse to make a contract with the key finder, but those kind-hearted usually make an exception and form a life-lasting bond, as seen with Arthur Moshiyoto and Voda, the leader of Water Spirits.
As stated before, there are numerous spirits and their respective keys and their strength vary from almost useless to strong enough to conquer a Zodiac Spirit. Because of the difference in power level between two spirits of the same kind, Elemental Spirits are commonly classified through an alphabetical order, varying from E-Rank, D-Rank, C-Rank, B-Rank, A-Rank and S-Rank with E being the weakest and S being the strongest. The spirits are sorted into their categories based on their power level and endurance, as well the ability to remain summoned for a longer period of time. Ironically, C-Rank Spirits are the most common elemental spirits, the reason remains unknown.
E-Rank spirits aren't really combat-oriented, but their lack of fighting skills can be replaced with their cheerfulness and optimism. Their magical abilities are pretty weak, allowing them only to use the basics of their respective nature. As the rank grows, so does their power. C-Rank spirits are capable of putting up a proper fight with a silver-type key, while the S-Class ones' power is equivalent to those of a Zodiac Spirit.
Elemental Spirit Gate Keys (星霊関鍵, Seirei Sekikagi) are magical artefacts that can be used by Celestial Spirit Mages in order to summon an Elemental Spirit. These keys can be found almost everywhere in nature, as Elemental Spirits are dubbed as Spirits of Nature. Their design can vary from element to element; water spirits are said to have a light and delicate design, while fire spirits often have a wild note to them. This is most likely due to the fact that once a spirit is mature enough to be summoned, they create their own key which is correspondent to their element. The element leader assigns the name of the gate when it is open for the first time. The higher class keys are often embedded with colourful jewels and have a grandiose design.
One of the more known keys is the one in Arthur's possession, the Key of Never-Ending Water has an elliptical bow with many engravings resembling waves with a pair of handles, representing an upside-down urn. The stem seems to be unstable, representing the flowing water. It gets thicker as it reaches the tip of the blade, resembling a wavy sea, every detail on the key is coloured in silver, while the key itself is dark blue.