Let me see what I kind find. I used to have a couple of Xenogears websites.
http://www.xgam.org/xenogears/profiles/index.php?profile=characters&ID=id
Here's what Wiki has to say about Id:
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: Ego, super-ego, and id). 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 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 takes us into Dissociative Identity Disorder, or DID for short. This is also known as Multiple Personality 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. In some cases it has been documented that when various personalities assert themselves people have been known to change eye color in a blink [citation needed]. 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. Also, there is a scene in "Xenogears" 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.
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I'd also like to add that as Id is just another personality of Fei; Id has the power of Deus aka God in Xenogears. A man made God, but still God. Id is known as the Demon of Elru. He is extremely powerful and his attitude is to be feared. He destroyed the Lahan Village with a simple output of energy. In another instance he's seen destroying an army and even beating to near death Ramsus with little effort. -=Ramsus is a substitute contact created by Krelian meant to replace Fei and as such he is very powerful=- Id also survived getting a giant sand submarine called the Yggdrassil chrashing on him, lifted it and then threw it at Bart while saying "A ship thrown at me...that's called cheating. Have it back" Id has also been seen destroying Gears with his barehands. He was also responsible of the destruction of Solaris an ultra high-tech society that considers the people living on earth to be lambs. Those are the feats I can remember at the top of my head.