Shin Seiki Evangelion; an introduction
On September 13th, 2000, half of the population on Earth was destroyed by the Second Impact, when a meteorite crashed into Antarctica and melted the ice caps, thereby flooding the southern hemisphere of the planet. That was the official story at any rate, but the truth, known only by a select few involved in the cover up, was that Second Impact was caused by man's attempt to control Adam, the first Angel.
Our story starts in 2015, when Ikari Shinji is called by his father to come to Tokyo-3. He is instructed to become the pilot of an Evangelion, Unit-01, and although he is reluctant at first, he ends up accepting his new responsibility and joining NERV. From then on, Shinji and the other pilots battle the Angels that have come to attack Tokyo-3 in order to save humanity from a Third Impact.
The anime first aired in 1995, and created a new way to look at the mecha genre. Unlike most mecha protagonists, Shinji fears his mecha unit, and instead of bulky robots, the Evangelions were thin and in reality... they were not robotic at all. For more info, read the notes.
Saigo no Shisha; the anime
Nagisa Kaworu's appearance was in episode 24 of the anime TV series. He came in at a time when Ikari Shinji, the main protagonist, felt alone and uncertain. Right before Kaworu's entrance, Shinji had realized the truth about Ayanami Rei, whom he grew very close to, as well as seen Souryu Asuka Langley, whom he had a crush on, deteriorate as a pilot and withdrawing away from the world.
When they met, Kaworu paid Shinji compliments and was friendly to him. They talked more and Kaworu's words seem to echo the truth about what Shinji has always felt and feared. Later, to avoid contact with his guardian, Shinji asked to room with Kaworu for the night, and Kaworu would give Shinji a sense of security. In the short period of time they have known each other, Shinji has grown to feel closer to Kaworu than he's ever felt toward anyone. Kaworu would eventually tell Shinji that he loved him.
The truth about Kaworu's origins would soon surface, and Shinji would once again feel betrayed. Nagisa Kaworu was not human; he was the last Angel sent to NERV by its parent organization, SEELE, in order to force start Third Impact. But Kaworu was also tricked by the old men who made him, and when he saw Lilith deep in Central Dogma, instead of Adam, he knew. Facing Shinji, who was inside Eva Unit-01, Kaworu explained the truth about the A.T. Fields, and told Shinji he wanted humankind to live on. In order for humankind to live on, Kaworu must die, since Angels (who ate from the Tree of Life) and Humans (who ate from the Tree of Knowledge) could not both live. And so, Kaworu urged Shinji to live on, and requested his own death.
Adam's Descendant; the manga
The main series manga adaptation is by Sadamoto Yoshiyuki, and was published from December 1994 to June 2013. It follows the original anime series much closer than other manga adaptations, and in many ways can be viewed as the "canon" companion to the TV series. There are notable differences in the story, however, such as a shift in the timing of certain events, and a difference in the number of Angels that attack Tokyo-3. The biggest difference is probably how some of the characters are portrayed.
Shinji is much bolder in the manga than in the anime, and takes a more adversarial approach to his father, and in the way he deals with the stress he is placed under. His personality is also much less timid than the way he was portrayed in the anime, more in line with a typical mecha genre protagonist. His interactions with Kaworu are also markedly different. Kaworu makes an appearance much earlier in the plot timeline, and instead of offering comfort when Shinji is at his most vulnerable as he does in the anime, he steps into the role of the disrupter.
Naming and episode title
Nagisa Kaworu (渚 カヲル) is usually translated into "fragrant seashore." Kaworu is written in katakana, and (though both are pronounced the same) uses the "wo" katakana instead of "o." In the episode, his name can be seen written as KAWORU, so I'll stress that "Kaworu" is not the same as "Kaoru."
Nagisa, on the other hand, is written in kanji, and can be related to the 24th episode title, "Saigo no Shisha" (最後のシ者), which translates to "the final messenger" or "the last person to die," depending on how you read the shisha part. The Japanese title of the episode (since Evangelion had an English and Japanese title for each episode) uses katakana to write the "shi" part instead of hiragana, and when you put the shi (シ) together with the sha (者), they become the kanji nagisa (渚). This is why sometimes the Japanese title for episode 24 can be read as "the final seashore." Additionally, another phonetic play on words can be applied to "shisha," which can be written as 死者, meaning a dead person, hence "the last person to die." The chapter title for Stage 72 of the manga is also written this way, and the same interpretation applies.