Summary: Cloud is a broken puppet, Sephiroth is a god, Jenova is inhuman.
Cloud is a broken puppet. He doesn't know "no" or "willing" or "consent". Cloud only knows that his master's will is the defining pillar of his existence, that anything Sephiroth requests must be done, that what he'll be rewarded with for following orders and being good is an identity. In a burst of ink and pain, his master will mark him with a number. Cloud wants very much to find out his purpose- With a purpose comes the promise of knowledge of how to fix himself, and that promise is more beautiful than anything.
He doesn't always succeed with his orders, and when he fails the green and black in his mind half-drown him. It's all he can do to stay afloat in a sea of pain. Others never notice the difference with him, because it is only inside-Cloud that hurts. Outside-Cloud, the devil-may-care mercenary whose eyes sometimes have a glimmer of gold and whose hair may shine black for a moment, a trick of the light, acts just the same as he always does. Sometimes Cloud wonders if it's normal to be two or three separate people.
Cloud thinks Sephiroth is a benevolent master. How many people (Or gods, as the case may be) would put up with a broken toy?
Sephiroth is a man, inhuman, and perhaps a god. And yet, in his own way, he is a slave as well. He has broken, shattered Cloud as a servant along with the other, lesser, mindless half-creatures Hojo created. Cloud is most valuable because rather than break completely and be lost, he has bent to the tide and found refuge in insanity- Much like Sephiroth himself.
When Jenova decides something must be done, there is no contesting her will. Sephiroth knows this, and hates it.
Jenova is not human, nor animal, nor anything comprehensible to the minds of man. She is simply herself, a force of nature and wholly unnatural all at once. If some, and perhaps all, must die to give her dominion, so be it. Destruction is her purpose, no more and no less.
She watches her puppets hang themselves on their own strings, and smiles.