I know there is symbolism behind the hyena (descent into savagery) and the orangutan but what does the zebra represent about Pi or his character?

The zebra represents the danger of passivity. If Pi chooses to simply sit and wait, circumstances (and predators) will tear him apart. Therefore, that is not an option for him. He can't just draw back and avoid the tiger. He must enter into an active struggle for dominance with it, contesting for mastery.