Keep in mind that Gendo, Fuyutsuki are members of Seele, which is a cult (which uses loads of occult symbols, not least the triangle that is suppose to have 7 Hebrew letters engraved on it, but instead has seven eyes).
This symbol is taken from a resource on qabalistic magick. If you want the name of the specific name of the book then PM me, it would be irresponsible of me to write it openly.
The ring of names around the outside are the names of 72 (or 31, depending from which caste you call) demons, now, this system is very unusual because the idea is to draw out these demons and put them in a body, which is then tied to the magicians will or intent no matter what, usually there is a bargaining between the demon and magician to reach an understanding of what both of them are prepared to do for each other, but not in this case, the will of the demon becomes that of the magician who calls forth its being into a vessel.
Much like the enochian angels, these spirits are very uncommunicative and are not suited to conversation at all, a la Rei.
I think the part in the middle has something to do withâ€¦
â€¦ and the progressive geometry needed to manifest a soul from the void (and the map for biological growth).
Back to how Fuyutsuki made Rei, like I said in the first paragraph he says that Rei is a product of his despair, obviously his despair is centred around Yui. This is interesting because with the birth of psychology many people went back over occult texts to apply their theories about the psyche to the realm from which the above symbols are derived.
During recent years, many ceremonial magicians with a background in psychology have postulated that the demons of the ------ are the base and negative traits of the human psyche. While there is certainly a basis for this idea, it must be understood that this does not diminish the possible danger of incorrectly conjuring and commanding these forces. These "archetypes," if you will, are not simply within the mind of the magician. They are in the psyche of each individual, as well as in the collective psyche of humanity. To illustrate psychologically, suppose the magician invokes a demon representing anger and ferocity, with the intent of utilizing the demon's aggressive nature to assist him in obtaining employment. If the magician does not properly control his anger, it may break free and cause a severe imbalance.
The shape of Rei is a product of Fuyutsukiâ€™s despair over Yui, keep in mind all members of Seele deal with occult forces. Contained within this vessel is a spirit or demon which is either Lilith herself or, in the hierarchy, Lilith would be Reiâ€™s overlord (which can be interpreted as a mother, amu and ano). To sum that up, Kozo gave his despair substance --- which was Yui-shaped!