Nagisa Kowaru appears only in episode 24. He arrives to replace Asuka and to pilot her eva, unit 02. The only information anyone knows about him is that he was born on the day of the Second Impact, and that Seele sent him because of his abilities as a pilot.
Kowaru meets Shinji at the seashore, which is ironic because Kowaru's name literally means "Kowaru of the seashore." Shinji is alone, moping about in a state of depression, when he notices a figure singing "Ode to Joy." This mysterious figure is standing near him with silver hair and very light skin. He immediately speaks to Shinji, and even knows Shinji's name. While Shinji is still in shock, this mysterious person introduces himself as Kowaru, the 5th Child.
Later on, the two of them go to do some tests, along with Rei. NERV wants to see how well Kowaru synchronizes with his eva. To their astonishment, he is able to set his synch ratio at whatever level he chooses.
When the tests are over, Shinji is outside, listening to "Ode to Joy" on his walkman. Kowaru walks up to him, saying, "Hello. You weren't waiting for me, I hope?" Shinji actually blushes, stammering an "Uh no, i was just.." Kowaru launches another conversation with ease, despite Shinji's awkwardness. He speaks of home and happiness, spouting out various philosophies and ideas on life. At this point, the two decide to shower before going home.
At the shower, Kowaru says to Shinji "You are extremely afraid of any kind of initial contact. Do you fear other people? If you shut yourself off from homan contact, you will never be betrayed, however you will never escape that feeling of sadness. Humans cannot completely erase that sadness, because all men are fundamentally alone." Then, he puts his hand on Shinji's, causing him to jump. The two discuss going home, and they end up going back to Kowaru's room. While the two are talking on Kowaru's bed, he says "I think i may have been put on this earth just to meet you, Shinji Ikari."
Later on in the episode, Kowaru takes control of the eva Unit 02 without even being in it (a sure sign that there's something different about that boy). He employs his AT Field. His AT field, along with the fact that Unit 02 is being activated without a pilot, causes every alarm in NERV central to go off. Kowaru beings to move towards Terminal Dogma down a shaft, and Shinji is dispatched into pursuit by NERV. Shinji argues with Misato because he is in denial that Kowaru is an Angel that he must destroy. After Misato finally convinces him, Shinji is enraged.
"Shinji, you're late," Kowaru greets him as he sees his eva approaching. Shinji angrily yells at Kowaru for "betraying" him, and the two begin to battle, pulling out Prog Knives. Shinji discovers that Kowaru has his own AT Field with Unit 02 loses its grip on its Prog Knife, and it is stopped by Kowaru's AT Field before it can hit him. When Shinji cries out in shock, Kowaru calmly states, "This is the light of my soul... a sacred territory in which no one may intrude. Aren't you lilim even aware yet that your so-called AT Field is merely the wall that incloses every mind that exsists?" In response to his own statement, Kowaru then employs the most powerful AT Field ever detected, shutting off all of NERV's communications and scans. NERV is unable to see what happens next...
Kowaru, Shinji, and both Eva Units reach the bottom of the shaft, and Kowaru moves towards "Heaven's Door." Shinji (still in Eva 01) tries to stop him, but is blocked by Eva 02. Another battle begins, and another AT Field is unfolded, eating through the other AT Field.
Kowaru opens "Heaven's Door" by looking at it, and views what he believes to be Adam. He soon finds out that it is Lilith. At this point, Shinji has defeated Unit 02, and he crashes through to where Kowaru is. It is then that Kaoru realizes that merging with Lilith will destroy mankind, as well as Shinji. He decides to sacrifice himself so that mankind can survive. With Unit 01's hand, Shinji grabs onto Kowaru, holding him in his palm. A very long and suspensful pause is displayed as Shinji stares down at Kowaru. (I think it was around two minutes, but I could be wrong.) Finally, Shinji moves his hand, killing and decapitating Kowaru.
This is an act that Shinji constantly regrets, but he knows that it was something he had to do. This ends the story of Nagisa Kowaru, or Tabris the 17th Angel, who sacrificed himself for the survival of mankind.