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Angel/s is the name used in the story to refer to aliens from far off space. Deemed worthy of the name Angel due to them being far above the humans in both technology and abilities. Described as ascendant beings who had graced the earth with Eden's Seed (Dark Matter) from their world, which gave birth to not only the Second Christ but also the Needless later on.
The appearance of the Angel/s vary from from each known incident. What they do all have in common however is that they possess a seemingly human face somewhere in their body, some in fact have multiple ones. What they do look like however is more akin to the description in the bible. They look like monsters of Lovecraftian origin. On occasion the angel/s have been shown to possess 6 fingers on each hand, when the angel has hands.
They originated from far-off space. Described by anyone involved as millions of light years away from earth. Their environment is implied to be heavy with Dark Matter energy. They are speculated by Dr. Rikudou as being far more technologically advanced than humanity. In fact he theorized that humanity may have stagnated or had been capped in their ability to grow technologically due to their evolution. Specifically he pointed out that Humanities evolution into a five fingered and toed being hindered our (Humanity) chance to arrive at technological Singularity. Compared to the angel/s who are shown to have 6 fingers in each hand. Their possibility of having arrived at a technological singularity's peak that could provide him with the technology he required to save the love of his life is higher.
The angel/s shown so far appear to belong to their version of a military. it is unknown if their entire civilization is of a militaristic one. This is implied when Dr. Rikudou Gidou and Saten opened the dimensional gate, the Angel which came forth demanded to have it's "Advanced Party"/Scouts be returned to them as part of their demands; along with other demands, but was cut off by Adam Blade.
The Angel/s cannot sustain themselves in earth's atmosphere, without succumbing to weakness or even eventual death. The Angel/s require a constant supply of Dark Matter energy to maintain their vast abilities and sustain themselves.
They do have 2 countermeasures open to them. One is to hibernate deep within the earth, unclear if it requires a land with at least minimal levels of Dark Matter. Two is to possess or inhabit the body of a human to cope with earth's environment. Option one leaves the Angel vulnerable and possibly weakened. Option 2 led to the creation of The Second. This option however is simply a temporary measure. The human body is incapable of truly housing the energies of the angel and would eventually breakdown and kill the host (Human) and the angel along side it.
Another possible weakness is the Shield of Aegis. Due to the nature of the fragment negating Dark Matter energy, the user could theoretically suffocate the angel eventually. Not only that, The angel/s are unable to go against the defensive capabilities of the Shield of Aegis.