Kambei Shimada is the leader of the seven samurai: the first one recruited and one of the only ones who walks away alive at the end. We don't know much about his past before we meet him, except that he once served a daimyo (a Japanese lord) and survived a terrible defeat in which said daimyo was presumably killed.
And yet from that little acorn grows a mighty katana-wielding oak. That tells us a lot about this guy, and by extension, the other six samurai who he comes to lead. We know he's a man of principle (he has to be, or else he wouldn't be a samurai) but also that he's had to bend those principles to stay alive in this world.
Where'd you Pick That Up?
How do we know that? Because he's still alive instead of dying fighting to save his lord, which is what he should have done. We get the details in bits and pieces, mostly during Kambei's interactions with his old buddy Gorobei. They were there when the whole thing went down, and it looks like they watched a lot of their fellow samurai get killed. And yet, they didn't die along with their lord and they didn't commit ritual suicide (or seppuku) like their code tells them they should.
That gives him an air of sadness and resignation that colors his otherwise noble face. This man believes very deeply in bushido, the samurai code of honor that's supposed to inform their behavior, but he's also seen plenty of times when bushido failed: when its code didn't save anyone or make any kind of difference at all.
Even worse, he's had to acknowledge his own failings as far as the samurai code goes; he couldn't kill himself and was unlucky enough to let his lord's enemies do the job themselves. We can understand that. You can be a noble hero and still not want to die, but for him, it's a seriously big failing. A failing that he has to literally live with every day.
Doing What He Has to Do
So he's had to bend the rules, and maybe he's had to take a few shady jobs here and there just to keep from starving to death. That's a pretty big downer, but it also explains why Kambei decides to defend the village (and therefore give us a movie in the first place). If he were more dedicated to his position, he would refuse to help the farmers because it wouldn't bring him any honor. If he were more of a mercenary, he would refuse to help the farmers because they can't pay him. But they find him perfectly balanced between those extremes: a man of principle who humbles himself to give the peasants a break, even though he'll gain nothing by it and it might cost him his life.
That's par for the course for a ronin, at least an honorable one like him. But there's something else echoing through his actions, something more than just "I had nothing better to do." We don't know how long he's been out here, but it's probably been a while, and the farmers' dilemma gives him a chance to remember what it was to be a real samurai:
KAMBEI: The reward will only be three meals a day and the fun of it, if you want to put it that way.
He's understating his case here. The "fun" was something he used to dedicate his whole life to, and which we hope he was pretty darn good at. It's not every day he gets to plan a whole military campaign. It's mostly little things like saving that baby from the bandit in his first big scene: noble deeds (and maybe some not-so-noble ones) but nothing befitting a soldier and a warrior like him. Then suddenly this little mini-war lands in his lap: terrain that needs to be defended, enemy forces that need stopping and maybe a few colleagues who know what it's like to go through the ringer. Suddenly, the "fun" starts looking a lot more "fun" than it has any right to.
So forward he goes, looking for a little taste of his past and maybe a respite for his guilty conscience. He gets the first part in spades, but the second? Not so much. He's left with four dead buddies, another freshly minted case of survivor's guilt, and a long trudge down the road towards whatever else fate has in store. "The farmers have won. Not us," he mutters in the famous last lines of the film.
And this actually makes him a great protagonist for the Japanese people. Like him, they had to accept life after defeat instead of defending their country to the death like they were told they should. It was a big pill to swallow for a people so disciplined, but they had to do it… just like Kambei does. He tells them that he knows exactly how they feel, then gets on with embracing life the way they need to do too. They need to keep moving forward like he does, and maybe find their way towards a brighter future where their nobility of spirit will find a better way to shine.