Before we start, I'll have you know that this short essay (if it merits so grand a name) is not written by a Kaworu fan, as the following paragraphs will I believe show.
Okay, where do we start? The title is as good a place as any. Discovering how many fans Nagisa-kun has gathered, I thought he must at least appear in two or three episodes, maybe even enrolled as a student in Class 2-A with the rest of the Eva pilots. Imagine my astonishment when I watched episode 23 and there was still no sign of him - I'd already watched episodes 25 and 26 earlier, so I knew Kaworu wouldn't be there. Then up he pops in episode 24, only to have his head cut off at the end of the same episode. That's it? I thought. The guy just appears for one episode but manages to collect so many adorers? Hot damn. Now how is that possible?
I admit (neither grudgingly nor very willingly) that he is cool. For one thing, there is that smile that seems to conceal many meanings behind it. Oddly enough, there has been no droolworthy male chara in Evangelion (don't tell me you know someone who drools over Shinji) until Kaworu comes along. I have a friend who didn't think much of Kaworu, then, wham! she watched Eva again and fell for him. Another friend doesn't even like Eva (now how can it be!?) but likes Kaworu. Such was the strength of the magic he radiates.
He is one of the Shitous (I choose to use the Japanese word, since I believe it has a slightly different connotation from "Angels"), but he is also portrayed quite sympathetically. (You are delicate like glass and that earns my sympathy? In other words, I like you? Do those ring a bell?) Plus, he tries to penetrate Terminal Dogma with more tactic and style than any of his brethren - and has more success too. Cool.
And then there's Shinji (I just bet that never slips your mind). Kaworu is just about the only person giving Shinji the feeling that he is not a "worthless being", that somebody cares enough for him to actually want to be with him. Kaworu-kun, you hit the mark there.
As for me, I've made it clear to myself why Kaworu is so popular.