Kimahri is formally introduced to us via combat. A kickass FMV plays and then Tidus is thrust into battle with him, alone. I am uncertain as to why this happens, though. Kimahri doesn't seem to be particularly invested in Yevon's teachings; his interest extends to Yuna and what is necessary to protect and help her. Our initial introduction to Kimahri happens in the Besaid Cloister where he says nothing, and then outside the Temple where he just shakes his head at Tidus. At no point does he demonstrate any aggression towards Tidus, be it inherent or because of his breach of the teachings.

I have theories, but Kimahri's unprovoked attack seems really out of character. Perhaps this is a Ronso way of testing Tidus. Maybe he wants to see if he has what it takes to be Yuna's guardian. Maybe he is just suspicious and combat is a way to derive a person's quality. But after this event, Kimahri returns to being silent and shaking his head every time Tidus tries to talk, so it is quite strange.