Handle Bone (柄骨,Tsuka Hone?, lit." Bone Hilt") is a bone-shaped hilt given to a Samurai by a Holder as a symbol of their bond. When a Samurai claims a Handle Bone, the Holder becomes known as a Key Holder. The Handle Bone is usually stored in a compartment in a Samurai's forearm. It's also possible for a Samurai to have more than one Handle Bone.
With the Handle Bone a Samurai can access two abilities.
Holder Control - A Key Holder typically recognizes the wielder of their Handle Bone as their master, so they respond exclusively to their commands. They can also create Samurai Armor from the Key Holder's outer layer of flesh.
Samurai Soul Blade - A Handle Bone serves as a control device that changes the form of a Samurai Soul. By putting the two together, a Samurai can transform the Soul into a Samurai Soul Blade.