Who is The Sheep from A Wild Sheep Chase and what is their importance?

The sheep is yet another unseen character in the novel, but is just as vital as a main character. The sheep is attempting to take over humanity in a rebellion of ideals that would place it at the helm of power. Using humans as catalysts for its ideas, the sheep inhabits humans, invoking brain tumors that allow the sheep to take over the mind. In this way, the sheep controls the human, making it immortal. Once it leaves, however, the human is left to die, either immediately or following a lifetime of misery. It is only through the weakness of the Rat that the sheep is eventually killed, as the Rat commits suicide with the sheep inside him.