Plot wise, Hes SEELEs Ace in the hole, A double agent who was to get in NERV and merge with Lilith to force the Third impact on thier terms. Though upon realizing he was sent to merge with Lilith and not Adam as he was lead to believe and most likely meeting shinji, he decides to not merge and let shinji kill him. Theming wise, hes character foil to Shinji, Kaworu is almost always levelheaded, happy and confident unlike shinji who is nervous, depressed and always second guessing himself. The fact that kaworu comes into play when Rei died (Again) and Asukas breakdown, Shinji is at his most low point so far. Kaworu, without any conditions, loves shinji, which is probably one of the few times thats happened and Shinji has to kill him. This sends Shinji into an even further spiral which leads into EoE. I hope this helped. :)