This guy is the devil's representative on earth. Need we say more?
We need not, but we will. Mephistopheles is a Big Bad because he uses dirty tricks, like threatening to tear Faustus limb from limb if he repents (even though that's exactly what does happen to Faustus because he doesn't repent), to keep Faustus for the devil.
That doesn't mean Mephistopheles can't be a bit of a good guy, too. At the beginning of the play, Mephistopheles displays real sadness and regret that he's been kicked out of heaven, and encourages Faustus to turn back from his intended course before it's too late. All the same, once Faustus's bargain has been struck, Mephistopheles is the devil's best warrior for Faustus's soul. It's a dirty job, but someone's gotta do it.