Goethe's Mephisto is very different from the crude devil of medieval legend and the original Faust story. He is a cultivated, witty, and cynical exponent of materialism and nihilism, and preaches a sophisticated doctrine of philosophical negation. Mephisto's most outstanding characteristic is skepticism; the inability to believe in anything. Ironically, although Mephisto represents evil, he can also be an unconscious force for good. This is first indicated by his presence at the side of God in the "Prologue in Heaven," which implies that evil is an accepted and natural part of God's universal system. This view is emphasized by Mephisto's relationship with Faust. Through his unrelenting efforts to corrupt and destroy the protagonist, Mephisto forces him to react with positive action, and is thus the agent of his ultimate salvation.
Mephisto's specific observations about humanity and the universe are usually right, because it is easy for him in his role as a "cosmic outsider" to discern real faults in the established system. At the same time, however, his vision is narrow and his total outlook is wrong. As a result he never fully understands Faust, makes inadequate plans for the seduction of his victim, and is finally defeated by Love, a force which he never recognized or comprehended.
On another level Mephisto represents the negative elements in Faust's own personality. This is why the devil and his intended victim are able to remain so close throughout both parts of the poem, and why, at certain points, like the Walpurgis Night in Part One where Faust's evil side is dominant, Mephisto is able to come so close to winning him. Mephisto fails, however, because he cannot understand or appreciate the positive sides of Faust's character (or human nature in general), and does not attribute any powers of resistance or resilience to Faust in the struggle for his soul.