Like so many characters in this movie, Shino is defined by her relationship to others. She's Manzo's daughter, the one he wants desperately to protect from the samurai he asked to help them, and of course, she falls for the youngest hunkiest one (Katsushiro) in the bunch.
That lends her a tragic air, like so many other characters here, because there's just no way that this society is going to let her run off with a samurai. "I wish I'd been born into a samurai family," she sighs at one point. It's like Romeo and Juliet on steroids: not only would the families disapprove, but the whole darn country would too.
We see that first-hand when Manzo finds out what's been going on and publicly humiliates her in front of the entire village. It's a make-or-break moment for their quest and almost leads to the samurai leaving (or even being killed by the farmers). They struggle through it, but the damage has been done and Shino has to break off her romance with the boy she loves.
That comes to a heartbreaking full circle at the very end, as she runs by him to join the other farmers planting rice. She looks back at him like a woman in physical pain, then reluctantly turns her back on him and starts singing along with the rest of the farmers. She knows that her village provides the only safety and security she can hope for, and that she needs to leave this young samurai behind if she wants to live.
It's a tough break, but survival is the name of the game here. She may be a nice girl, but she has duties, and that doesn't include a boy from an entirely different caste.