Hero in KOF is Kyo. It always has been him. In all sagas.
Terry and Ryo (as well as Mai, King, Yuri, Kim, Athena, Clark and Ralf, Andy, Joe, Robert, etc) are BASIC ones. KOF started as a crossover between SNK titles, mainly FF and AoF. So these characters are the base of KOF, the participants who give popularity and visibility to KOF. Those which everybody wants to see the tournament (I'm talking from a story point of view). They are not really related to Orochi, tresures, power, etc. but they are famous. And they are always there, like some kind of champions of KOF. Specially Terry and Ryo, since they are main characters of FF and AoF.
Iori and K', even Ash, are the typical guys who started as enemies and then became something more. But they have the same profile. Cool, handsome, hating everyone, ignoring everything. Ash is a bit different because he looks girly, but is the same character profile after all.