The reader is Junketsu, plain and simple, he is always attempting to get off Satsuki, but his body naturally is trying to devour her as he does. He has the ability to speak, but can do little to get Satsuki to listen. And he can only gain control when he puts everything he has into fighting Satsuki's will. He finds it embarrassing to be worn, and the forced lifefiber synchro thing effectively is like being assaulted to him.