Soulmates, he's told, are your other half. Perfect for you and you alone. The one who will understand you better than anyone else. One particularly amused old woman had told him in whispers, like it was a secret, that the Kami had separated soulmates from each other because they were in fear of the power two souls, twisted together in all the perfect ways, could wield. She told him that soulmates are to be cherished above all else. After all, if they put something above their perfect other half, then they are clearly less then human. Monstrous in a way not even shinobi, for their bloody way of life, can match. What your soulmate touches looses all colour, a trail of grey and black and white to lead you to them. When they die, you are left in a colourless world. Many, she says, blind themselves to stop from being reminded eternally of all they lost. Others are killed by their sorrow.
"Promise me," the old woman had said one day, solemn and serious. It was that that had caught his attention, rather than the words. He had paused, turning his full attention to her.
"Promise me," she said again, "that if you find your soulmate, you'll hold on to them. Don't let them go. If you have your chance and you let it slip through your fingers, you'll regret it, no matter your reasons and excuses." It's a promise he makes like lead in his stomach because he knows he won't be able to honor it.
The old woman dies when he is just seven.
Now, he has no one to whisper him stories of soulmates and perfect other halves. His mother is long dead, his father uninclined. He is the one the tells the stories to his brothers because he is the one that knows the stories, that clings to them like a lifeline.
He protects his brothers, mostly. He's their father's soldier so they don't have to be. He trains and fights and kills so that they have the choice not to. His brothers will always be more important than him. More important to him than anything, even his soulmate. Especially his missing soulmate.
He doesn't begrudge his soulmate for not being around. He can't, in good conscious, when he's missed so much of the other's life, as well. Sometimes, though, he thinks it would be easier without soulmates. He would never wish not to have his soulmate, regardless of his wandering thoughts. Soulmates are precious.
Just... perhaps not as precious as his living, breathing family. A family he knows and is oh so lucky to have.
It's just his luck, then, to find his soulmate on the battlefield. It's just his luck that his soulmate is the brother of his rival (almost friend, he can see the fondness hovering just out of reach, ready to crash in when this meaningless war is finally, finally over). He knows, then and there, his soulmate can't know.
Not until Butsuma and Tajima are killed and fed to the worms. Killed, because he is not so lucky to think that either of them would die another way. He realizes that he doesn't mind if he is the one to put the pair out of commission.
Perhaps that makes him the monster, the demon, that others always call him.
Perhaps he knows, now, why the old woman had told him to cling to his soulmate once he found them.
Perhaps she, too, had had a soulmate on the other side of the battlefield.
Perhaps she had seen them cut down before her, by her own family.
He won't allow the same to happen to his own soulmate. They will have their peace, even if he must give his life to get it. Anything to keep his family and his soulmate's family from killing each other.