Kai dislikes a lot of things about Rei.
He dislikes a lot of things about Takao and Max too, but he is able to ignore that. In fact, he doesn't really care if they're there or not, because they are lost causes. However, Rei is not a lost cause, and so Kai dislikes Rei for many reasons. But it's not Rei.
It's Kai who has the problems.
On Mondays, after practice, Kai dislikes the way that Rei slumps on him, leaning on him for support after a gruelling session of attacks, counterattacks and strategies. He dislikes the feel of Rei's chest pressed towards his side, feeling the sweat and thumping heartbeat of Rei. But Kai always trains him harder the next week. One day, Kai reasons, Rei will not have to depend on him so much. Stamina building. Yes. That's it.
Similarly, Kai hates it afterwards, when Rei insists on taking a shower. He dislikes the way that he has to share a hotel room with Rei, and so he can't escape when Rei comes out, dressed in nothing but a pair of boxers, skin still gleaming wet. Hair is undone, flowing free, and Kai dislikes the secret smile that appears on Rei's face. Next time, Kai thinks, he'll confront Rei and ask him to change rooms with Max or someone. But there'll be another next time. And another.
And when they share the hotel bed, Kai dislikes the way that Rei sighs softly in his sleep. He dislikes the way that the moonlight falls onto his face, and he hates the way that Rei's hair seems to tangle everywhere, managing to weave itself into Kai's grasp every single time.
There are times when Kai can hear Rei exhale with annoyance, see Rei roll his eyes patronisingly and pout ridiculously. Each time, Kai looks away, hurriedly. He dislikes a lot of things that Rei does, because Rei usually does them around Kai.
But Kai thinks it's healthy to dislike.
After all, it's better than not caring at all.