Zeus probably does not appear in the play because of rules forbidding the presentation on stage of the supreme god. Regardless of whether this is the actual reason, Zeus's absence contributes significantly to the image of him as it is developed throughout the play. First, this absence allows for Zeus to be represented by his servants, who leave an unfavorable impression. Kratus, Force, and Bia, Violence, convey some fairly obvious hints about Zeus through their names alone. Clearly the tyrant rules entirely through intimidation and punishment rather than cooperation and friendship. Stress is repeatedly placed on the idea that Zeus rules by his own laws and does not answer to anyone for what he does. As a result there are two possible ways to view Zeus's rule in relation to justice. Either he is simply not just, since he clearly does not follow any rules, or he is equivalent to justice because he sets the standards of right and wrong.
The possibility of Zeus creating his own and the only possible justice in the course of his rule leads us to a second important feature of Zeus's absence. He is allowed to be represented by servants who suggest that Zeus's law is the only possible law. Though this may seem reasonable since Zeus is the greatest of the gods, Aeschylus undermines our faith in Zeus as a just god by contrasting the total lack of pity in Zeus's servants with the sympathy experienced by Prometheus's friends. The suggestion is that right and wrong exist apart from Zeus and he, as a new ruler, has not yet learned this. This may be why the newness of Zeus's reign is repeatedly mentioned: it is to show that he has not yet fully adjusted to the role of just ruler, and the current lack of a fit between him and justice is responsible for the conflict between him and Prometheus.
Finally, we should note that Zeus's absence in the play adds to a feeling of isolation surrounding him. Not only is Zeus still removed from justice by the arbitrary nature of his laws, but also he seems removed from others. Since, as evidenced by his servants, Zeus demands complete obedience in body and mind, he can have slaves but cannot have friends. Zeus is left isolated from both Prometheus and the audience. The isolation reinforces the impressiveness of his power, which would be diminished if we were to see him. At the same time, however, this isolation makes Zeus look very lonely. We see this through a contrast with Prometheus, who closes the play in the company of his friends. Zeus's servants, on the other hand, never leave with friends. Kratus leaves with Hephaestus, who dislikes him, and with Bia, who doesn't talk. Hermes leaves all alone. Zeus's servants live entirely in his shadow. As far as we know, Zeus's character does not change in the course of the play. He certainly becomes concerned when he hears Prometheus's prophecy, but his servant at the end, Hermes, is just as arrogant and uncompromising as his servant at the beginning Kratus, so we may assume Zeus has not undergone any serious change.