A Samurai in Training
If Kambei is Obi-Wan (the crafty old master who no one sees coming) then Katsushiro is Luke: the bright-eyed young kid who thinks the old man is super-cool. Katsushiro comes from a noble lineage and has the bearing of a samurai, but not the training. He's out in the world to find a master, and with Kambei, he's kind of hit the jackpot. Oh, the old codger's a little reluctant at first, but Katsushiro's bright-eyed enthusiasm (and a little help from Kikuchiyo) wins him over and the young man is in the club. "Please take me as one of your disciples," he earnestly tells the older samurai, and spends the rest of the movie showing us that he means it.
In a lot of ways, that makes Seven Samurai a coming-of-age story, at least as far as he's concerned. And as anyone who's seen a coming-of-age story knows, that road gets far rockier than our young hero (or heroine) has in mind. Interestingly enough, we don't see a whole lot of his combat skills here, though he's clearly got them. His training comes more in the form of military tactics, giving him first-hand experience with wartime combat and showing him how the code of bushido, which he's clearly familiar with, operates in the real world.
Along the way, he shows his bravery and skill, as well as developing a little hero worship of Kyuzo, who he sees as the perfect embodiment of the samurai in his quiet frame. "I've always wanted to tell you how great I think you are," in a voice normally saved for superheroes.
He gets to learn about the bad stuff too: like losing your buddies or watching the people you're her to protect turn on you. He's also a sensitive lad, so the brutality of those lessons hits him pretty hard. Even so, he's up for the task, and prepared to accept the sacrifices that come with being a samurai.
Or he would, if Cupid hadn't fired a dart at him the size of an intercontinental ballistic missile.
Love in samurai stories never ends well. They marry for political expediency, they go to geishas to have someone sympathetic to listen to, and real, actual romance doesn't enter the picture. Or at least it shouldn't. When it does, it tends to throw the samurai for a serious loop and it typically ends in tears for everyone involved.
So it is with Katsushiro, who, being the young buck that he is, soon catches the eye of a pretty farmer's daughter named Shino. Their romance is what you'd expect: super awkward and with a lot of running and hiding in a bashful manner. But it also drives a rift between the farmer's and the other samurai—the peasantry wants outsiders to keep their mitts off the womenfolk—and threatens their ability to work together in the face of a common foe.
But it's more than that. Even if there weren't any armies of bandits barreling down on them, samurai aren't supposed to date peasants. Like ever. And if Katsushiro is going to be the samurai he wants to be, he needs to respect that part of the code. Breaking it would mean living in that littler village forever, and thus never becoming what Katsushiro wants desperately to become.
That's the big sacrifice he has to make if he wants to continue on his path. And surprisingly enough, we don't know for certain what he decides. He's left standing in the road like a lovelorn puppy, watching Shino trot off to the rice paddies with a look on his face like he's going to throw up. He might decide to follow her, but he knows what he'll be giving up. And staying on the samurai's path means walking away from her. We suspect he'll stay the course and leave her behind, but even if he doesn't, fate is asking him to pay a super-sized price for it.
But hey, nobody said the hero's journey was easy. Whatever he decides, Katsushiro will learn and grow from it, which is the whole point of the exercise from a Campbellian perspective.