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 with Lucifer,
And are for ever damned with Lucifer.
We learn that Lucifer's minions are always on the lookout for souls to snatch:
...when we hear one rack the name of God,
Abjure the scriptures, and his Savior Christ,
We fly in hope to get his glorious soul;
Nor will we come, unless he use such means
Whereby he is in danger to be damned.
Notice that man has to act first; the devils are not sent unless the soul is such that it can be damned. The devils "stoutly...abjure all godliness" and "pray devoutly to the Prince of Hell."
We know that, at least for Mephistopheles, earth is part of hell because it is not heaven; anything short of that is hell. If Faustus had the will and the ears to listen, Mephistopheles tells him that heaven is better than hell in an attempt to be certain that Faustus does want to sell his soul to Lucifer.
Why this is hell, nor am I out of it.
Think'st thou that I that saw the face of God
And tasted the eternal joys of heaven
Am not tormented with ten thousand hells,
In being deprived of everlasting bliss?
The tormented devil makes it as clear as he can that there are only two choices, heaven and hell, and that hell is a place of torment, even for them.
Within the bowels of these elements,
Where we are tortured, and remain forever.
Hell hath no limits, nor is circumscribed
In one self place, but where we are is hell,
And where hell is there must we ever be.
And to be short, when all the world dissolves,
And every creature shall be purified,
All places shall be hell that is not heaven.
We also know that the devils have no real power over Faustus if he chooses to repent, so they use whatever fear tactics they can think of to keep him from doing that. One method is to make certain Faustus does not believe the truth that God will forgive him any sin, including this grave sin of signing his souls and eternity over to Lucifer. Another is used by the bad angel when he tells Faustus that the devils will tear him into pieces if he repents.
Lucifer himself makes an appearance in this play, but his words and demeanor are more like a boss who is checking in on his employees--and his future employee--rather than some frightful presence. It is the tortured soul and person of Mephistopheles whom we see the most.
Records from the Elizabethan period suggest Mephistopheles was played in the costume of a dragon.
It has been said that performances of the play were so terrifying that during the 17th century audiences believed that the devil actually appeared among them.