A reflection; why he was important
It's hard to argue against Kaworu's importance in the story. Although quite brief in both anime and manga, Kaworu's presence affected Shinji in slightly different but important ways. In the anime, he provided Shinji with a sense of belonging and unwavering attention. In the manga, however, he confused Shinji to no end, and made Shinji unwittingly care for him despite all the discomfort. There was of course the fact he was the last remaining Angel. His defeat would complete NERV's purpose and destroy the last remaining road block to SEELE's plan for Human Intrumentality, a plan SEELE was willing to change in order to defeat Ikari Gendou's own plan.
There was also the fact his introduction in the anime fascinated fans, especially those of us who had interest in yaoi or boys love. Even though Sadamoto changed this in the manga, his take on the "love" between Kaworu and Shinji still gave plenty to talk about—a kiss to block Shinji's mouth, and a more detailed (albeit less receptive) declaration of love? And both anime and manga versions left behind a character filled with mystery.
Years and years ago, when I first found out about Shin Seiki Evangelion, Kaworu's part in the anime enthralled me. I'll be honest, as yaoi fandom back then was small (this was early 2000s), his hinted (romantic or not) feelings toward Shinji were enticing. But as I watched the whole series many, many times, Kaworu's character became more important than just a symbol for my yaoi fandom. When I first realized his change in personality in the manga, I didn't like it at all. But as usual, it grew on me, and with his final choice, he became much more important to me than he ever was before. As selfish as he may have been, his choice to die on his own terms gained my respect. His childish personality was also a refreshing twist to the benevolent angel trope.
However, I also have a will. And I can choose by my own will to defy destiny. Nagisa Kaworu, Stage 73
I don't really believe death should be feared. Maybe I'm just young and foolish, but as all living beings must die, I think rather than dreading the inevitable, we should make something of our lives now, so that when we die, it's with the knowledge that we made the best of the time we had. I believe Kaworu died at the end having made the best of the time he was given. He chose to leave his mark with Shinji, rather than going down as a tool for SEELE. He'll be remembered not as SEELE's messenger, but the Final Messenger by his own right, the last remaining Angel.