Personally I think alot of the KaRe thing comes from the english dub of Beyblade being edited because the producers thought Kai and Tyson came across as too couply in the original. Can't remember who found out that little tidbit, either Glay or Ranma, I think. So in the english dub Kai seems more annoyed by Tyson, and more at ease around Rei.
That said even with all the editting, I was bombarded with blatent tyka innuendo from the first episode of season one, so I don't really get the KaRe thing either.
Could be because I always pictured Rei as being off limits via Mariah.