Id, a form of the subconscious mind according to psychologist Sigmund Freud, is Fei's destructive side. This coincides with Freud’s principle of the “Id” versus the “Super-ego” (see “Id, ego, and super-ego”). The Id is the “instinct” side of the subconscious: it craves self-gratification. The Super-ego, in turn, is the logical and less instinctual side. Moreover, the Ego is the side that decides the balance between the Id and the Super-ego.
In this case, Fei Fong Wong is the Super-ego. Id wants to destroy everything and cause chaos, while Fei’s Super-ego side is stable and peaceful. The third, recessive persona existed as a catalyst and the balance between the two: the Coward, which is a representation of the aforementioned “Ego” in Freudian psychology. In Fei’s case, this manifested as Dissociative Identity Disorder. When Id asserts himself, Fei’s hair eyes and skin change color. His brown hair turns crimson, his brown eyes turn bright yellow and his dark tanned skin turns a sickly pale. It has been claimed that when different personalities emerge in some people their eye color changes, however, there is no evidence for this nor is there a known mechanism for such a drastic change to take place.
The seeds of Id were planted because of the trauma that Fei suffered as a child at the hands of his mother, Karen Wong. As a small boy, Karen was possessed by Miang. Fei was taken to testing labs and hurt mentally and physically. When he dissociates, he becomes insane, destructive, and extremely powerful. Because of this, Fei was indirectly responsible for his mother’s death, as well as the destruction of Lahan Village. As an assassin under Grahf, he was responsible for the annihilation of several civilizations; one in particular being Elru, Domina's home country.
Also, there is a scene in Disc 1 where Fei sees into his subconscious mind. He sees the “Coward” personality and the “Id” personality. The Coward and Id walk off, then Id says to Fei, “Sorry, but you can’t come.” Whenever Id surfaced, Fei’s Gear Weltall transformed into Weltall-Id, a blood-red Gear of incredible power with six glowing wings made of energy. Even without his Gear, Id is monstrously powerful and fully capable of destroying Gears bare-handed.