Kitsune have something called a Hoshi no tama, star ball or kitsune ball. They are white and/or gold orbs. The orb is like a kitsune's life force so they hide it, usually in their mouths or on their tails. If someone manages to get a hold of the orb, they have control over the kitsune.
What is the extent of that control? Could one keep a kitsune until they die and the kitsune reclaim the ball?