Manzo is one of the farmers who goes out in search of samurai to save his village from bandits. He's not the most enthusiastic about it either, issuing proclamations of doom and doing a heck of a lot of whining as well. "We were born to suffer. It's our lot in life," he mutters at one point, and it's hard to say that he's wrong. Even so, while other members of his village try to make a difference, he seems almost petulant in helping them.
The reasons why become obvious soon enough. (Well, except for an abject fear of dying, which makes itself clear from the very beginning.) Manzo has a pretty daughter, Shino, and he worries constantly about her virtue. Strange samurai showing up with their dashing swords and codes of honor could lead her into a compromised position, so he tries to hide her beauty by making her dress like a boy.
It doesn't take.
The samurai are onto his little reindeer games from the beginning, and sure enough, one of them falls for her. Manzo kind of loses it on her, and in front of the whole village to boot, which brings all of his fears about death and bandits and samurai to a screamy weepy crescendo. He's more or less ordered to deal with it, but ultimately gets the final word as Shino stays with him and the village instead of running off with her lover.
In that sense, he's typical of the people in the entire village: scared of their own shadows, but finding a way to stick it out regardless. As Kambei says, they're the real winners of the conflict, a point brought home by the sight of Manzo's smiling face at the end. There's no comeuppance for his cowardice, no twist of fate to make sure he gets his. He benefits the most from hiring the samurai, even though he did everything he could to derail the whole thing. This is not a fair world, his victory tells us. It's just a world where people do what they have to do to survive.