I believe she'll become sadden and unable to smile anymore with the loss of her friend Joshua. She will, however, take on her duty and meet it with no joy.
Eventually, she'll end up on the battlefield herself because she'll refuse to left behind where its safe. Eventually, she'll encounter one of the enemy's soldiers who does smile and somehow brings a smile back to her face. After which Yuki will remember the peace she sought before and strive for peace once more between her kingdom and the empire. That enemy soldier will do what she can to assist Yuki.
That's a theory on how it will go anyways.