Adam; progenitor of Angels
SEELE revealed to us that Kaworu carried Adam's salvaged soul in Stage 71. While all Angels originated from Adam, the first Angel, SEELE's words could mean more than that. Kaworu's only given data, besides his name, was his birthday—the day of the Second Impact: September 13th, 2000. Ritsuko, who was in on much of the secrets at NERV, told Shinji of man's folly (quoted below). For what they tried to do to Adam on that day, they were punished, but it's most likely whatever part of Adam was saved, his soul was given to Kaworu. SEELE must have made him then, knowing what the Dead Sea Scrolls prophesized. They probably provided his body, making Tabris the only Angel with the shape of a human being.
Once... people did find god, and in their joy, they tried to possess him. That is why they were punished fifteen years ago. Akagi Ritsuko, Stage 70
In each Angel "attack," while they gave the impression of an invasion, their only purpose was to return to Adam. Kaworu also holds this instinct, but his given human shape made his thinking different. He wasn't mindless in his pursuit of completion, though perhaps we only believed that because of his human shape, which enabled us to empathize with him more. His curiosity made him unique. For much of his life, he was held under close watch by SEELE, waiting for the right time to carry out his role.
But the god they had tried so hard to find disappeared. So they tried to resurrect him on their own. That is Adam. And from Adam, in the image of god, they made human beings. That is Eva. Akagi Ritsuko, Stage 70
With her knowledge, Ritsuko revealed that mankind had tried to revert Adamn back to his embryonic form, before the other Angels could awaken. This was what Commander Ikari Gendou possessed, stolen and implanted on his hand, and this was what made the committee so worried. Since Ikari Gendou had his own plans, but held a key part of the Human Instrumentality project, SEELE had to push Ikari's hands into initiated Third Impact, on their terms.
For his part, Kaworu was not as surprised as his anime counterpart to find the giant being held in Central Dogma to be not Adam, but Lilith. Seeing Lilith there only reinforced Kaworu's speculation that SEELE was sending him there only for their purpose. And with that, Kaworu decided to take back some control over his life.