Kyuzo is the strong, silent type: the kind of guy who shows up in a huge number of American Westerns. He doesn't say much, but he's got mad samurai chops, as we see in the spectacular opening duel where he's so certain that he'll win, he tells his opponent there's no point. "If I use a sword, I'll kill you," he tells him. "It's stupid."
That's not bravado: he measures up the other man, applies basic samurai awesomeness to the equation, and reaches the sober conclusion that there's no way this jabroni can take him. Naturally, the jabroni samurai persists and naturally Kyuzo cuts him in two. He takes neither joy nor sorrow in it. He's just doing what he has to do.
Kyuzo quickly becomes a poster boy for what samurai should be. In addition to being able to cut people up like fishbait, he's silent, honest, honorable and follows orders without question. He stands in stark contrast with Kikuchiyo, who's loud, sullen and disobedient. Kurosawa actually goes out of the way to show us how a "proper" samurai (Kyuzo) steals a rifle from the bad guys and how a Johnny-come-lately does the same thing.
Katsushiro worships him of course, in part because he understands why Kyuzo is so good at what he does. It doesn't help him in the end, when he's killed by getting shot by surprise. It's another sobering reminder in this film that being everything you're supposed to be can't always save you, and that sometimes reality just trumps whatever code you live by, even one as strict as bushido.