Thanks for the suggestion. My idea was mostly to let the DM and player decide what is it (My default idea is it is like a gem with the face of the Captive into it, but maybe it is one of the weapons or an obsessive living poem living inside the head of its bearer), and since these two, the captive and the bearer, act as one and can't be separated unless the Captive dies (Unless the DM and player decide into other stuff) I did not give it an inventory slot, but since I plan to sligthly improve it someday, will give it a very deep thought...
Thanks again for your feedback ;D