Mephisto
Mephisto, sometimes using the name Mephistopheles or even Satan (among many others) is a powerful demonic super-villain from Marvel comics and is one of several such demons that feed off humanity's belief in The Devil so as to masquerade as Satan himself. In reality however Mephisto's origin seems to lie with the primordial Elder-Gods that ruled earth before being banished by Atum the God-Eater, the exact nature of this origin varies with story to story but tends to vary between the old gods becoming corrupt or the demons being formed by the already plentiful corruption found in the Elder-Gods.
At any rate Mephisto now rules over his own inferno dimension, commonly called Hell but not related to the Biblical realm (though to the average human being it may as well be as it is equally as horrific), within this realm Mephisto rules supreme with only demons such as Blackheart (his own son) being capable of challenging him there.
The Actual Lucifer
Mephisto is one of three devilish hell lords in the Marvel Multiverse who masquerades as the true Devil Lucifer, along with Satannish and Marduk Kurios. They do so to increase the fear others have in them and the number of their followers. The actual Lucifer himself has not been seen for centuries, his only involvement in recent times being his interaction with the Ghost Rider, but even the credit of that interaction is often stolen by Mephisto who masquerades as him again. While the other two are just impersonators, Mephisto's relation to the true devil is often questioned, and may possibly a fragment or extension of the actual devil's will, or a direct creation of his. Overall the true answer is uncertain, but it is quite evident now that the actual Lucifer is apparently the keeper of Ghost Rider's soul and it appears that his lack of interaction with the mortal world for the past countless centuries was due to being permanently imprisoned in Hell at some point.
Eventually, Lucifer tricked Ghost Rider into escaping from hell but in doing so took Lucifer with him, thus freeing him. In transit to Earth Lucifer split into 666 different fragments each occupying a body of the recently deceased. Lucifer goaded Blaze into killing the individual bodies. As each body died, the fragment of its strength would go into and increase the power of the remaining fragments with the plan for Lucifer to be completely reunited and at full power on Earth when he would have enough power to kill the Ghost Rider. Before his plan could be completed Ghost Rider made one of the hosts brain dead tricking Lucifer into fighting him at only half of his power. Once that host was defeated and before it could full materialize in the last host, the last host was killed sending Lucifer back to hell and imprisoning him once more.