Speaking of the Kairi theory, KH Insider had a forum thread about how Kairi may have escaped. Some of them chose a more humorous theory: She kicked him in the nads. Bless their hearts. However, some cite the novels in saying that Saix beat the tar out of Axel and nabbed Kairi himself.
Hmmm...at face value, that makes some sense, and I'm sure that Vexen, Zexion, AND Saix fans would have just LOVED that. However, it would only make sense, BEFORE Sora reached Betwixt and Between. However, Saix encountered Sora after the battle of 1,000 Heartless and well before B/B. At that time, Axel looked perfectly fine(in more ways than one, huuuur), so he couldn't have been beaten then. Unless, Saix was so sure that he would get Kairi that he made it sound like he already had her.
Axel encounters Sora, but Kairi is somewhere else, Saix catches Axel and chases him. When Saix catches up to Axel, he takes Kairi, leaving Axel to die. That would explain why Axel sacrificed himself for Sora--he was already dying, so he decided to go on his terms(which is very Axel).
Blagh. I like my truck theory better. The thing is, the Saix beating Axel only comes from the novel, so it isn't "game canon". Because, think about it, wouldn't something like that be a sight to behold for some of the fans? You can bet that some people would have LOVED it, while Axel fans would be dying.
I just think that Kairi got away before Hollow Bastion(she kicked him in the nads or something), Saix gets her, Axel goes to Sora, Saix finds him, ??????, Sora finds Axel again, Axel dies.
Blafg, I guess we'll never really know since not everyone is going to read the novel/manga. I guess this is where "using your imagination" comes in.
Personally, I've always wondered why Mickey gets in front of Sora before the fight with Demyx.