Age: 15
Sex: Male
Birthday: September 13, 2000
Relations: Shinji Ikari (Ally)

Kaworu Nagisa is both the 17th Angel and the Fifth Child. Although classified as an Angel, Kaworu is similar to Rei Ayanami in being a Seed of Life that inhabits a human body. Despite his human form, Kaworu does not consider himself to be human, since he refers to humans as "the Lilin".
Kaworu appears humming the fourth movement of Beethoven's Ninth Symphony, the same music that plays during his descent into Terminal Dogma and so serves as a leitmotif for his character. The Ninth Symphony itself incorporates an adaptation Friedrich Schiller's Ode to Joy (Ode an die Freude), a poem dedicated to the unity of all men under God. The moment Kaworu opens the doors to Terminal Dogma is in synch with the line "Und der Cherub steht vor Gott"- "And the Cherub (Angel) stands before God".
Kaworu's body appears to have been created as a result of the Contact Experiment with Adam, wherein DNA from an unknown human donor dove into and fused with Adam's flesh, explaining Hyuga's discovery that Kaworu was "born" on the day of Second Impact. SEELE salvaged Adam's disembodied soul into Kaworu, whom they may have recovered in embryonic form in Antarctica.
Kaworu's origin from Adam, only intimated in the anime, is flatly stated in Sadamoto's manga.
Since Kaworu was delivered to NERV by SEELE, it is believed that Kaworu has been living under SEELE's surveillance prior to his appearance in the series.

Kaworu is brought to NERV to replace Asuka, who is no longer able to pilot Unit-02. He is only able to pilot Unit-02 because its soul is in hiding. In his first test, Kaworu immediately achieves impressive synchronization results. It later emerges that Kaworu is able to achieve whatever level of synchronization he chooses with Unit-02.

(Source: Evangelion Wikia)