I of course respect the modern au, and there are plenty of good ones for Kainora, but I just think it's super important to Kai as a character that he practically went on a suicide mission for the Air Nomads, and for Jinora especially. There's a wrenching character arc here somewhere about who's only selfish because he feels so abandoned because he's so mistrustful and even grief in that but hides that under a smile, and there's something genuinely but subtly spiritual that's happening here.
Everything about Kai is amazing. He had so much potential for rich character development. Personally, I love how all his survival instincts morphed into selfless willpower. The prison stuff, the air bison, freakin P’Li, and the book 4 stuff. He did all of that without hesitating. He’s such a confident and fearless character. Like, I worship him.