When Mephistophilis first appears to Doctor Faustus in scene 3 of Christopher Marlowe's The Tragical History of the Life and Death of Doctor Faustus, he's so hideous that Faustus demands that Mephistophilis "return and change thy shape" and come back to him as a "old Franciscan friar" (scene 3, lines 24–26). Mephistophilis does as Faust demands and returns to him as a Franciscan friar.
MEPHISTOPHILIS. Now, Faustus, what would'st thou have me to do?
FAUSTUS. I charge thee wait upon me whilst I live,
To do whatever Faustus shall command.
(scene 3, lines 36–37)
Mephistophilis responds that he can only do what Lucifer commands "and may not follow thee without his leave" (scene 2, line 43), but Mephistophilis has just changed his appearance in response to Faustus's demand, seemingly without Lucifer's knowledge or consent.
FAUSTUS. Did not he charge thee to appear to me?
MEPHISTOPHILIS. No, I came hither of mine own accord.
(scene 3, lines 45–46)
Again, Mephistophilis acts without Lucifer's permission. However, Mephistophilis apparently has "standing orders" to act independently:
When we hear one rack the name of God,
Abjure the Scriptures and his Saviour Christ,
We fly in hope to get his glorious soul.
(scene 3, lines 50–52)
After Faustus questions Mephistophilis about God throwing Lucifer and his followers, including Mephistophilis, out of heaven, Mephistophilis—who has come to claim Faustus's "glorious soul"—implores Faustus to "leave these frivolous demands" (scene 3, line 86). Mephistophilis knows what hell truly is, and he seems to want to spare Faustus from the torments that await him if he proceeds with his bargain with Lucifer for his soul.
In scene 5, Mephistophilis, the servant of Lucifer, continues to warn Faustus against the torments of Hell. Faustus persists, however, and Mephistophilis almost immediately changes tactics and tells Faustus that if he makes this bargain for his soul, then Faustus will be "as great at Lucifer" (scene 5, line 55).
When Faustus's blood congeals in his arm and he's unable to sign his contract with Lucifer in blood, Mephistophilis gets a "chafer of coals" to warm Faustus's blood, remarking gleefully, "O, what will not I do to obtain his soul?" (scene 5, line 77). This is the same Mephistophilis who was warning Faustus against making such a bargain just a few scenes ago.
Later in the scene, after Faustus signs his contract with Lucifer for his soul, Faustus requests that Mephistophilis give him a wife. Mephistophilis tells Faustus, "I prithee, Faustus, talk not of a wife," (scene 5, line 150), but he then provides Faustus with a devil dressed like a woman—who Faustus immediately rejects—as if to prove his point that Faustus shouldn't ask for a wife.
Mephistophilis's inconsistent and conflicting behavior towards Faustus continues throughout the play, particularly when Faustus asks for what Mephistophilis thinks are "frivolous demands" for pleasure of the flesh and crude magic tricks to mock the Pope.
Mephistophilis grants all of Faustus's "frivolous demands" but belittles Faustus for asking for them. Mephistophilis believes that Faustus is wasting his soul asking for meaningless pleasures.
However, when Faustus asks to know something meaningful or substantive—regarding the movement of the heavens or universal truths, for example—Mephistophilis refuses to respond to Faustus's questions, either because he cannot or simply will not answer. Mephistophilis is essentially failing to fulfill the contract that Lucifer made with Faustus that Lucifer give Faustus the knowledge of the universe.
Mephistophilis does whatever it takes to secure Faustus's soul, including granting Faustus's final wish:
That I might have unto my paramour That heavenly Helen [Helen of Troy].
(scene 12, lines 81–82)
Ultimately, it is Mephistophilis who escorts Faustus to hell, despite Faustus's last minute cries to be delivered from eternal damnation.
[The clock strikes twelve]
FAUSTUS. O, it strikes, it strikes! Now, body, turn to air,
Or Lucifer will bear thee quick to hell.
[Thunder and lightning]
O soul, be changed into little water-drops,
And fall into the ocean ne'er be found.
My God, my God, look not so fierce on me!
Adders and serpents, let me breathe a while!
Ugly hell, gape not! come not, Lucifer!
I'll burn my books!—Ah Mephistophilis!
[Exeunt with Mephistophilis.] (Epilogue)