The key spirits are six powerful spirits which, according to legend, when are all together can save or destroy the world. Though it is later revealed that they form the ultimtate spirit Tusker. As the series progresses the key spirits become the focus of the show's plot. The six key spirits are:
The strongest member of the 6 "Key Spirits" of the show. At first, Amil Gaoul appeared as a turquoise being who could summon two feathered wings to create its powerful wind-based attacks. After it started morphing, it first awakened as a Phoenix; then, when it had completely awakened, it returned to its humanoid form, this time with the addition of feathered, golden armor. In its final form, Amil Gaoul is able to open its eyes. Its rightful, chosen owner is Zed. Amil Gaoul chooses Zed because they both desire an end to the meaningless fights between the nations. Since Amil Gaoul is the strongest of all 6 key spirits, it cannot be taken from Zed unless Amil Gaoul chooses to break that relationship.
In the very beginning it was split up with Zymot having half and the Kalbu-fu the other half. Hugh was able to combine the two together to form Pronimo in the invasion of Kalbu-fu. Later when Hugh was killed, Zed's mother took Pronimo. She then lost it to Jeem, who gave it to Task's savior, Mirette. After being unable to control Tasker, Noa took it from her.
Noa's key spirit that he received from a group of Nomads, shortly after entering the world. Sachira was taken by Mirette so she could awaken Tasker, but when Mirette couldn't control Tasker, Noa took back Sachira.
Neotopia's key spirit. It was hidden under the statue being built as a symbol of power and discipline at Neotopia, but when Neotopia fell under Task's control, it awoke, and was obtained by Sara, Zed's mother. Later, Sara lost Menardi to Jeem. Jeem gave it to Mirette to summon Tasker, but when she failed to satisfy Tasker's desire for a strong host, Noa took it.
Tusk's key spirit, which remained dormant in Tusk's savior, Mirette, until all other key spirits were in the vicinity. When Mirette fails to summon Tusker, Noa takes Jumamis from her.
Key spirit of the Seekers. Sagiri was given Shadin when she returned to the Seekers, as she was viewed as the savior. She gave Shadin to Zed in order to help him find Noa. Later, Jeem took Shadin from Zed and gave it to Mirette. When Mirette fails to summon Tasker, Noa takes Shadin from her.
In the anime, Tusker is the result of the 6 Key Spirits brought together. Tusker is said to be the power that created and can destroy the world. It has the ability to control all spirits (Ex-Machina was the only spirit that couldn't be controlled by Tusker). Tusker needed to merge with the true savior, who appeared to be Zed (Zed, Mirette, Noa, and Sagiri are considered saviors because they were chosen by key spirits), and its goal turned out to be the creation of a world that only included spirits. Zed was able to stop Tusker due to the influence of Jiko, who appeared in his conscience - separating himself and Amil Gaoul from Tusker. Once Zed separated Amil Gaoul, Tusker was thrown off balance and defeated. Tusker's abilities consist of close combat fighting, the manipulation of other spirits, the original 6 key spirit's abilities, and the unleashing of meteor showers that destroy the land.