previously you had been a shinobi who had betrayed his nation and you escaped, that was when you met Sasori and he turned you into his puppet but for some strange reason you still had your own consciousness and could move at will and it took Sasori a while to assimilate this... Although you had your own consciousness and could leave if you wanted, you decided to stay with Sasori since, well, you had nowhere to go and it was your best option. One day Sasori was fixing an arm that had fallen off but you didn't stop moving until Sasori got fed up and told you in a brusque and angry tone. —"Can you stay still?! I'm trying to fix you."