Mephistopheles (n.) is not merely repressed sexuality but power too — practically the whole life of Faust except the part taken up with thinking, split off when Faust identified with one function.
Mephistopheles (n.) is really more positive as a personality than the vapid, unthinking careerist Faust — the supraordinate personality in distorted form.
Mephistopheles (n.) is Goethe's version of the alchemists' familiar Mercurius — the slippery, evasive, poisonous spirit whose aunt is the snake.
Mephistopheles (n.) is Faust's evil set up outside himself as an alibi in case of need — he 'knows nothing of what has happened' to Philemon and Baucis.
Mephistopheles (n.) shows how far the devil had shrunk by Goethe's day: a personal familiar, the mere shadow of the struggling hero, an appendix to psychology.
Mephistopheles (n.) is the diabolical aspect of any psychic function that has broken loose from the hierarchy of the whole psyche and enjoys absolute power.
Mephistopheles (n.) is Mercurius in Jung's reading of Faust: a chthonic spirit akin to Wotan and the Devil, entering as a dog born of chaos and fire and becoming Faust's familiar.