Christopher Marlowe presents many forms of the devil in Doctor Faustus. We meet Lucifer, the devil himself; Mephistopheles, one of his chief devils; and various "bad angels" and lesser devils/demons. All of them are characterized as evil, though perhaps not purely evil as one might expect to see them.
We know that Mephistopheles can change his appearance, as that is one of the first things Faustus asks him to do since he finds the devil's appearance unpleasant. Later in the story he and Faustus both change their appearances into the likenesses of priests when they visit the Pope.
We know that Mephistopheles is subservient to his master, Lucifer, and can only do what Lucifer commands him, including following Faustus. This means Faustus can ask some things of the devil and they can be done immediately; other things cannot. Mephistopheles is not an autonomous being with free will like we all have. He and his fellow devils are
Unhappy spirits that live with Lucifer,
Conspired against our God...
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