The Second Christ or more commonly referred to as "The Second" is the first and most powerful of the Needless whose name and origin are unknown. He was the only Needless to possess all of the Fragments in their complete entirety, with maximum output.
The Adam Project was an attempt to create a another Second Christ through cloning. Adam Blade and Adam Arclight are the only successes of the project and because of this, look exactly like the Second Christ. The Second Christ obtained his power by fusing with one of the angels who came through the gate when the Black Spot was created. His remains were consumed by Arclight before the start of the series.
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The Second's death was initially wrapped in mystery, firstly assumed as a conspiracy by other nation's or Japan's Government assassinating him due to various reasons. The truth is he died due to his human body unable to maintain the raw power and dark matter energy of the Angel that possessed him.
His remains were confiscated by the Japanese Government and was subsequently used in the Adam Project. Whatever remained of his remains at the time of Adam Blade and Adam Arclight's creation, were subsequently absorbed into the body of Adam Arclight.