In Act 1, Scene 3, as he contemplates making a deal with the devil, Faustus remarks, "Had I as many souls as there be stars, / I'd give them all for Mephistopheles" (1.3.100-101). It sure sounds like he really loves the guy.
And you know what? He really does. Mephistopheles is a source of never-ending delight for Faustus. He brings the guy women and wealth, enabling him to conjure the spirits of Alexander the Great and Helen of Troy, and taking him on a spur of the moment, round-the-world vacation. Oh, and to top it all off, ol' Mephistopheles takes the doctor on a trip to the stars, just so he can learn the mysteries of creation. That's some tour guide.
It's no wonder Faustus is so willing to sign over his soul to the devil in exchange for twenty-four years of having Mephistopheles as his servant. The dude has skills.
As a spirit, Mephistopheles has some major supernatural powers, which he uses to keep Faustus in line. He manipulates our main man out of repenting by threatening him whenever he thinks about God or heaven, or calling on his devil friends to distract Faustus and win him back to the Dark Side. It's almost like some sort of strange form of reverse psychology. He manages to make Faustus feel guilty for thinking good thoughts about God.
That's a master Baddie if you ask Shmoop. But what else should we expect? Mephistopheles makes his allegiance to the devil abundantly clear the moment he first appears, telling Faustus, "I am a servant to great Lucifer / And may not follow thee without his leave. / No more than he commands may we perform" (1.3.38-40).
Mephistopheles serves Lucifer. You don't get any Badder than that.
Yes, Mephistopheles's allegiance is clear. But that doesn't mean that it's all flowers and rainbows. No one ever said the guy was happy about serving ol' Lucy.
See, the problem is the dude is always in hell. Mephistopheles remarks to Faustus, "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?" (1.3.75-78). Mephistopheles clearly knows what he's missing. And knowing what he's missing sure does make him miserable.
Before the bargain with the devil is struck, he even advises Faustus to turn back, as if to say, don't do it, if you know what's good for ya. As a fallen angel, Mephistopheles is someone who knows firsthand both the joys of heaven and the torments of hell, and at least at the beginning of the play, he's clear about which is the better choice.
So even though he spends all his time tempting Faustus towards the Dark Side, Mephistopheles also stands as a warning to Faustus. While he tries with all his might to win the scholar's soul forever, he also pushes Faustus toward Good, by showing just how Bad bad can be. As he counters when Faustus doubts the existence of hell: "I am an instance to prove the contrary, / For I tell thee I am damned and now in hell" (2.1.131-132). Mephistopheles is a constant reminder of the torments that await Faustus in the afterlife. At the same time, he's the character who works the hardest to ensure that Faustus will eventually encounter those torments. Hey, what can we say? He's a complicated guy, with a complicated name.