Amil Gaoul

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. (Source: Wikipedia)