Ghost Rider

The Devil, Mephistopheles, sends his bounty hunter of the damned, the Ghost Rider, to retrieve the contract of San Venganza for control of a thousand corrupt souls. Seeing that the contract would give Mephistopheles the power to bring Hell on Earth, the Rider refuses to give him the contract.
In 1986, Mephistopheles (Peter Fonda) reaches out to 17-year-old stunt motorcycle rider Johnny Blaze, offering to cure his father's (Brett Cullen) cancer in exchange for Blaze's soul, to which he hastily accepts. The next morning, Blaze awakes to discover that his father's cancer is cured, but he is killed that same day in a motorcycle stunt in which he falls into the ring of fire he is jumping through. Blaze accuses Mephistopheles of causing his father's death, but Mephistopheles considers their contract to be fulfilled and promises to one day see him again.
21 years later, Blaze (Nicolas Cage) has become a famous stunt motorcycle rider known for surviving numerous deadly crashes. Blaze meets his lifelong sweetheart Roxanne Simpson (Eva Mendes), now a news reporter, whom he abandoned after his father's death. He convinces her to attend a dinner date. Meanwhile, Blackheart, the demonic son of Mephistopheles, comes to Earth, along with three fallen angels who bonded with the elements Air, Earth, and Water. They are tasked to find the lost contract of San Venganza. In response, Mephistopheles makes Blaze the new Ghost Rider and offers to return his soul if he defeats Blackhearts, though he is also told he has no choice in the matter. Blaze is driven straight to the station on his "first ride", where he transforms into the Ghost Rider and kills the Earth Angel Gressil. The next day, he meets a man called the Caretaker, who seems to know all about the history of the Ghost Rider. He tells him everything that happened was not a dream and that it will happen again, especially at night when he is near an evil soul.
When he arrives home, Blaze finds Simpson and reveals himself as the Devil's bounty hunter. Unconvinced, she walks away in disbelief. After a brief imprisonment for the murders that Blackheart committed, Blaze kills the Air Angel Abigor, and escapes from The Police, before returning to the Caretaker who tells him of his predecessor, Carter Slade, a Texas Ranger who hid the contract of San Venganza. Blaze returns home to find that Blackheart has killed his friend Mack and has taken Roxanne captive, threatening to kill her if Blaze does not deliver the contract. Blaze tries to use the Penance Stare on Blackheart, only for it not to work since Blackheart has no soul to burn.
Blaze returns to the Caretaker and obtains the contract. The Caretaker reveals that he is Carter Slade. Slade tells Blaze that he is more powerful than his predecessors since he sold his soul for love as opposed to greed. Slade leads Blaze to San Venganza, then gives Blaze a lever action shotgun before fading away.
After killing the Water angel Wallow, Blaze gives the contract to Blackheart. He quickly transforms into Ghost Rider in an effort to subdue Blackheart, but dawn arrives and he is rendered powerless. Blackheart uses the contract to absorb the thousand souls into his body. He attempts to kill Blaze, but is distracted when Simpson uses Blaze's discarded shotgun to separate them. After Blaze tries to kill Blackheart with the shotgun, he moves in and uses his Penance Stare to render him catatonic, burning all the corrupt souls within Blackheart, which is what he had hoped for.
Mephistopheles appears and returns Blaze his soul, offering to take back the curse of the Ghost Rider. Determined not to make another deal, Blaze declines, saying that he will use his power against him, and against all harm that comes to the innocent. Infuriated of being robbed of the power, Mephistopheles vows to make Blaze pay, but Blaze tells Mephistopheles that he is not afraid. Mephistopheles then disappears, taking Blackheart's body with him. Later, Simpson tells Blaze that he got his second chance before sharing a final kiss with him. Blaze then rides away on his motorcycle, turning into the Ghost Rider.

Question: Who is Mephistopheles?
Answer: The Devil