This is Au during the Fulbringer arc. A long time ago Shinigami knew what it meant to be soul mates. As time passed they have either forgotten or buried the truth. But with the loss of Ichigo's Shinigami powers Shiro takes it upon himself to help Ichigo in the only way he can when he has no direct access to his wielder anymore. And he gets something out of it too.

When Zangetsu travels back in time after a terrible war against Aizen and the Quincy, he does everything in his power to make sure that Ichigo doesn't grow up alone. No matter what, Zangetsu will support his King.